So I am a beginner to D&D and I chose to be chaotic neutral for the freedom of basically doing whatever I want and I was wondering if I was starting to be "that guy?" By that, I was hoping you know that people could tell me what counts as "that guy."

One example of a problematic incident was when I walked into a magic shop and the shopkeeper has no powers and seems to be well fed and moderately wealthy. My character suggests tying him up and taking his stuff (including a bag of holding, the cause of all this). I want to let him go afterwards with about half his stuff. I was planning to blame it on the rising number of cultists who worship Tieamont and desperately need to arm themselves.

The other PCs are either lawful or good and are mostly ok with the actions but the other players don't like my plan and seem irritable when I talk about it. They say they don't really think that that is the point of our campaign so it really isn't a good decision.

How do I fix this issue with my group?

Consider the tone of the game.

The first thing to do is make sure everyone is on board with the tone of the game. You said you wanted a character that could do whatever they wanted. Sometimes that is perfectly appropriate. IN a light-hearted game meant to have plenty of comedy it may be appropriate to do truly random things because they are funny, or silly, or just amusing. That can be great, as long as everyone is on the same page.

Similarly, in a "villainous" or "evil" game, deciding to rob a shop-keeper may be completely appropriate. If the entire group is not "villainous" or "evil" though such an action could be expected to create inter-party tension and should either be avoided or done in such a way as to avoid the conflict.

Consider the impacts of your actions on the group and the world.

In any game, you should consider the impact of your actions on your group. That is the main way you avoid being "that guy". That doesn't mean you avoid all inter-group disagreements. Inter-party conflict can be a story in itself. But it means you do things thoughtfully, keeping the impact of your actions on the group and the story in mind. Depending on your group dynamics, it may mean discussing possible ramifications out of character before you do something in character.

Also, in a game meant to be played seriously (not all are), consider the way the world will react. Robbing a shopkeeper that has enough connections and resources to hire a mage to cast divinition to help with the investigation may mean that your character is wanted, while a less well off shopkeeper might prove a far safer target. Again, this doesn't mean you necessarily need to avoid doing it. Sneaking in and robbing a target is a time-honored part of many RPGs for characters with certain moral persuasions. But it does mean that you need to plan for both the likely later consequences as well as the immediate security needed to accomplish the robbery. It also means you need to consider how that fallout could affect the larger team if they are not all of the same moral persuasion.

Consider the spotlight

Also, when considering the impact of your actions on your group, remember to consider how the spot-light is shared. One possible approach to resolving the particular problem described might be to handle the robbery as a solo operation or with a smaller sub-group. But if you do that during the main groups gaming time that means that, unless handled with enormous care, you will be taking most or all of the spot-light for an extended period. It isn't always wrong for one player to have the spot-light for even an extended period, but it needs to be balanced out in some way for the others and it should be handled with the impact on the others in mind.

Even if you convince the entire group to come, it still means that the focus of the game at least for a while is on your chosen side-quest rather than the main plot. If the others aren't particularly interested in that side-quest it still shifts the focus in a way they may not like. That could be part, possibly the main part, of why they get irritable when you fixate on the plan in question. For busy people, and everyone at least thinks they are busy, game time can be precious and even having it side-tracked in game can be annoying.

Don't let alignment be a straightjacket.

Remember that alignment is meant mostly to be a description and not a straightjacket, especially in later editions like 5e. A lawful good character may be able to justify stealing under extreme circumstances and a chaotic evil character may well find charitable actions appropriate under some circumstances. Alignment can be changed and even without formally changing it, it should not be viewed as something which constrains a character's particular choices in a moment.

This especially applies when you think playing out your alignment in a certain way will lessen the overall fun for the group.

Be Careful with a "Do Whatever I Want" Character Dynamic.

On the one hand letting your character dictate everything you do is problematic and means potentially giving up all decision making as a player, which starts to become more an acting exercise than a game. However a character who acts with no personality beyond what the player feels like doing at the moment is hardly a character at all. There is a reason the whole inspiration mechanic was introduced to reward players who make a character-based decision that is suboptimal as a game choice: it makes for a more immersive and interesting game experience through good roleplaying, at least theoretically.

It may be that, do to an inconsistency of behavior, your group is finding your character underdeveloped and having trouble investing in him enough to accommodate his wishes, or just objecting to a somewhat "powergame"-ish move or a "murder hobo" playstyle. I am not trying to criticise or mischaracterize your playstyle (your fun is not wrong!), but simply draw attention to how it might not be meshing with this particular group.

If it is a matter of them just not liking this dynamic in your character there are ways you might be able to persuade them otherwise. You might try to make your character's audacious behavior as entertaining as possible, or make him a sympathetic person trying to do good but forever drawn to temptation. It may even be that playing up the "do whatever he wants" to the hilt will work for them just fine if you just provide compelling enough character reasons for why he thinks and acts the way he does.

You also seem to have a conception of a chaotic neutral alignment that many people, including perhaps your group, would find dubious. It is hard to argue that robbing and holding captive an innocent person for pure personal gain is not evil (if that is what you are doing). Although moral neutrality has a lot of leeway, a person who frequently does evil things is evil in many people's book whether they do good at other times or not. Reasonable minds vary wildly in how they think alignments should be defined and applied. This may be rubbing other players the wrong way or just not something they think their characters would reasonably participate in (even if they would tolerate you doing it solo).

The group might feel different about the plan if there is a greater good or at least some greater adventure purpose behind it (ie: "We need these health potions to survive our showdown with a legendary dragon!"). It sounds like there isn't, or they don't think there is. Bear in mind that there is a substantial risk of the group becoming wanted criminals if they pursue your plan, and although your "blame it on the cultists" scheme is clever (and fun!), it may well come down to passing a single deception roll, which is no guarantee of it working. Many DM's like to come up with complications for this sort of thing because it generates more adventure. This may be a simple matter of the risk v. reward not seeming worthwhile to them, in which case the best angle might be to figure out ways to eliminate (or at least persuade them that you have eliminated) the risk.

Once again I am not trying to suggest that you are playing "wrong" or that you can not have many rewarding experiences playing with this group even if they play a little different than you. Nor do I really know much of anything about you or your group and the social dynamics that actually have arisen therein in or out of game. But I do think you have to account for the possible issues above, which may strike with or without anyone realizing. You should talk with them about what is going on out of and/or in character to make whatever precisely is making you and possibly them unhappy with how things are going. You don't necessarily need to make a big deal about it, some groups need only a slight sharing of perspectives to come to a quick compromise.