what should i do ?
Read the help?! Seriously. You are mucking around. AE is not a realtime app and requires to render RAM previews for smooth playback.
I highly recommend that third link Mylenium gave you: http://blogs.adobe.com/toddkopriva/2010/01/getting-started-with-after-eff.html It will help A LOT. Is there a lot of stuff there? Yes. But that's because AE is a very powerful and thus very complicated piece of software. There is a lot to know before you can even do basic things.
thankyou for the quick reply! reading the links now
yup, I am still trying to figure out how stuff works here in AE, its really complicated ehh