Chess Tournaments Without Studying Theory? I have played chess since a very young age and my tactics are very developed in my opinion. However, I never bothered to read chess related books about opening or related theory. This often leads to interesting games as I don't follow well known openings which throws off my opponents. Would it be a good idea for me to enter a tournament with an entry fee in the hopes of winning prize money? Or would I get completely crushed by my opponents? I am roughly a 1900 ELO rated player - checked online.
There is nothing wrong in entering a tournament if you have the time and if you can afford the entry fee.

However, I think you should really consider your first tournament as a discovery experience and enjoy the pleasure of chess and the atmosphere of the tournament rather than worry at once about winning prize money.

Whether you can win something depends a lot on the specifics of the tournament, the field, and of course your own level. I wouldn't put too much faith in online estimates of your strength: real-life chess is quite different. Playing your first tournament will actually give you a much better evaluation of where you stand vis-à-vis competition chess, and the experience will probably make you pretty stronger for your second one!

In a typical Swiss-system open tournament, you won't get crushed in all your games: whether your 1900-elo online estimation translates to real-chess 1900, or 1700, or 1500, there are many players of every level entering such events, and there will almost certainly be some opponents that you can beat. Probably you will get crushed in a few games, but not throughout the tournament, and you will have your revenge in others.

Some tournaments have prize money for unrated players. This may be a nice plus at the end, or a goal to aim for if your are well-placed before the last couple of games. But don't worry about it at the starting line, concentrate on your chessboard!

Finally, studying chess, reading books or training is up to you. It certainly doesn't hurt to play over some grandmaster's games collection, for instance. In any case, don't overestimate the importance of opening theory: it does help, but you wouldn't be the first tournament player to reach decent results without delving into it. 

Welcome in the world of competitive chess and have a nice tournament!