I play on a minecraft server and I'm trying to figure out what is causing lag

Its the speed of your internet connection an sometimes the distance from the server to your computer most of the time but if you are on a local server it could be your computer needs an update or there are to many programs running when you are playing check any forums about the game you may find a lot more solutions to your problems

Your internet speed is going to be the bottleneck - specifically, your upload speed - long before anything in the game itself is causing lag.