How to aim FPS games precisely with a controller?

I bought a chinese generic controller with vibration (you know, it looks like an xbox controller) and I've never used a controller before, always used keyboard and mouse( and I must say, keyboard and mouse are the best! Keyboard and Mouse FTW!) So now when I try to play Modern Warfare II or First person shooters, i cannot point the crosshair correctly to the enemy, it always moves a little bit to the left or right or up or down, and sometimes it hits though. I know practice makes perfect, but still any sensativity issue or any other tips? Thankyou.