You're not giving us any information to work with to help you. We need to know a lot more to help you. Please provide answers to the questions listed here: "FAQ: What information should I provide when asking a question on this forum?"
I'm going to take a guess, and I'm guessing that you are previewing by pressing the space bar. Don't do that. Use RAM preview to get real-time playback of previews.
Start here to learn After Effects: http://adobe.ly/bjBT3P
Try a ram preview. If it's still not real time then set composition resolution to auto and size to 50% and try again. If that still doesn't work, set size to 25%.
If you are still having real time playback problems tell us more about your system. If you do not know what a ram preview is click the link in the post above this one.
Many thanks. Much appreciated.