First of all, you must understand there's a difference between actual lag, and simply having a poor framerate. If you are already familiar with this difference, I apologize ahead of time.
Lag is when there's a poor connection between your computer and the server. This usually results in delayed controls, but in particularly bad cases everything will occasionally stop and you'll be unable to move or do anything in-game, but you'll still be able to use the menus.
Poor framerate is not lag, or at least not lag in the sense of a poor internet connection, but I see it referred to as lag all the time. If movement appears choppy but everything is otherwise responsive, you just have a poor framerate, and should probably either turn down your graphical settings or get a new computer.
I hope this helps at least somewhat. You didn't give a lot of specific information about your problem, so this is about all I can do.
