So when i drive alone in LFS then i can drive without lag but...If i play in servers then when im near to 3 or more cars then there is horrible lagg and it makes me drive out of road.If i drive near to 1,2 car then there is nothing.
So what causes it?Internet connection?Computer RAM?

Internet connection. Look at the little green bar on the bottom elft of the screen when playing online. If it's long it means you have a high ping and probably lag.

Most of time its still green and bretty close to red...