Resigning the game prematurely is the only 100% guarantee that you won't ever win against your opponent...
At your level, strange things happen in games, and players make simple mistakes left and right. There's no reason to be resigning randomly just because your position is a little uncomfortable, like you did in your last game. Chess requires some ability to play difficult positions, and not just give up whenever you don't feel like playing anymore.
Also, don't resign until it's clear that your opponent is winning. Just because you missed a simple tactic (which happens often at that level) doesn't mean your opponent won't see it.
Also, the fact that you missed a simple tactic doesn't mean that your opponent won't miss a similar tactic!
(1) Grammar skills. A sentence should not end in a preposition.
(2) Knowledge of the reasons behind the opening moves. If an opponent goes "out of book," in this case meaning too far off from the most popular line, then there is something wrong with the move. If you understand the reasons behind the opening moves then you can figure out what is wrong with the move, then refute it, if not immediately then over the course of the game.
I get confused and thrown off easily (usually by weird opening moves) and I have trouble finding what moves to do (I simply don't know what to do or don't have good reasons for my moves). What should I do to combat this weakness? My calculation is decent and I feel like this is holding me back from beating my opponents.