I just got TF2 2 days ago, It's a very good game but I need some tips
like when i'm a spy and cloacked it takes a while until i'm able to backstab a person when just uncloacked, while other spies can do it while still cloacked :S
It's just a matter of practice. I'd suggest searching the forum for any threads with tips for playing spy. There have been at least half a dozen of them since I joined up.
You might also want to check out the TF2 Wiki. Each class has its own page with tips and strategies specific to the class. Other than that, it just takes some time playing to get the hang of what's going on and what you should be doing.