It's very hard to know the cause without knowing more about your system and your project. If your computer is a few years old, that could be the problem. Also, the dimensions of your project (eg: 1920x1080 pixels, etc) will affect its speed. However, if you're previewing at a lower resolution, it should work fairly well. Do you have your project previewing at Quarter resolution, or was that just its original setting? At the bottom of your Viewer panel there should be a popup that says "Auto" or "Full", "Half", "Third" or "Quarter". Make sure it's set to Half or Quarter and see if that helps.
You have given us almost no useful information, but let me take a guess based on the assumption you've been using AE fr about two weeks or less:
Hit zero on the numeric keypad. Wait a bit. Happiness ensues. Unless, of course, you're doing something ridiculous like using 8K footage on a laptop.
I also recommend a detailed read of the Viewing and Previewing section of AE Help.
AND DISREGARD ANY MENTION OF THE SPACE BAR>
Hey there tkenney,
Did you ever get going with After Effects? Let us know if you have any questions.