Plan out chest routes with at LEAST 4 or 5 tiers 2s on them, and plenty of tier 1s for you to choose from as you go along on it. You can watch an abundance of chest route videos on YouTube or in the Media Section of the forums. Make sure you have at least 2 on each map you play. If you don't have a chest route on a specific map, then don't play it until you do. Next, make sure to play defensive at first; don't make decisions that have large chances of you failing, for example attacking a team of good players head-on or going on a chest route that is very known and popular or with someone that you know has a weapon. Also, you will need to have good frame rate to play and PvP well. Before, when I used my Windows XP to play, I couldn't do a thing, and got 27 wins in about 5 months. Don't be like young me. When I got my Windows 8, I was getting 40-50, and with Optifine, I boosted it to 70-100. Finally, practice is key. If you warm up and practice before you play MCSG, you won't make silly PvP mistakes. I usually go on Roxbot or the Battlegrounds to practice until I am getting 3-4 kills per death, or in the case of Battlegrounds, winning 80% of the rounds (which I do, anyway).
Once you follow these steps, you will slowly improve your skill, maybe without you knowing it. Soonafter, you will win more often and without much trouble, as you've already adapted.