No offence, but you really haven't told us a thing. Please give a detailed explanation of the problem you're experiencing, the content of your projects, the results you expect, and the results you're getting etc.
well i have a video which i recorded. I click the play button and it lags very badly. so i click RAM preview and it doesnt fully render it, it goes to about half way and then just plays the vid. I dont udnerstnad why the whole video wont render.
After Effects doesn't work like a non linear editor. Content must be loaded into RAM to play back. Play back is limited to the amount of accessible RAM in your system.