There are three things you can do here.
First of all, if your mouse has DPI controls, you probably want it set to a low number, like 400 or 800. These are typically optimal numbers for FPSes. Second, in the Windows settings for mice (as well as whatever options for drivers you may have installed), you want to set sensitivity to the middle option so that there's no interference from there.The in-game options should be your last layer of customization.
I posted about this earlier in this thread, but chances are your sensitivity is several orders of magnitude too high for appropriate FPS play if you've just got it set to what is most comfortable for using a computer. You want to give yourself as large a margin of error as possible, and this means setting the sensitivity fairly low. How low you should go is subjective, but it's worth noting that strong FPS players generally have their sensitivity set such that a complete sweep from one side of the mousepad to the other results in one to two 360 degree turns. Chances are, if you're using unadjusted mouse settings, you're probably getting five, ten or even twenty 360 degree turns by going from one end of the mousepad to the other, meaning you must make extremely minute hand gestures to get correct aim. It is not hard to imagine why this would be a fairly significant handicap.
With this in mind, the sensitivity to shoot for is generally the lowest possible value that you can stomach while still being able to move comfortably. Set your sensitivity to an amount and see if you can still move around the stage easily, and then go lower until you can't. The reason that this is a good method is that movement really doesn't require precise mouse input, but aiming does; therefore, your config should strive to make those precise inputs easy.
A note on acceleration: I'm not taking into account players who use acceleration in this post. Acceleration makes calibrating sensitivity far more complicated. There are benefits to acceleration but there are also significant drawbacks; it is best to get used to using a linear sensitivity setting first before you experiment with acceleration. The advantages of acceleration are fairly obvious -- you can get faster turns and more comfortable movement without sacrificing aim -- but many players find it difficult or impossible to be consistent with acceleration enabled and therefore play without it. YMMV but I consider the use of acceleration to be fairly advanced so you shouldn't worry about it for now.