The easiest way to create a slow-motion in After Effects is to right-click on your layer and choose time->time stretch and enter a stretch factor of 400% to play the video at only a quarter of its original speed.
To get a good quality and avoid jerky playback, make sure to choose the proper frame blending options. Experiment with the different blending options to see which one looks good and doesn't take too much time to compute.
Alternatively, you can also use the Timewarp effect.
Film with the highest possible framerate
The general problem with slow-motion is that you want to show more pictures than you actually filmed. Say you want to slow down your clip to 25% speed (i.e. time stretch factor 400%). This means in-between each two pictures of your film, After Effects needs to insert three other images and guess their content as good as possible (by looking at the frames before and after them). Of course, it would be much better, if After Effects wouldn't need to guess, but if you had actually filmed these frames instead. If you, for example, shoot with 50 frames per second although the final clip should have only 25 frames per second, you get a 50% slo-mo for free. Even if you want to slow down to 25%, only one additional image (instead of three) needs to be reconstructed in-between the existing frames. Hence, for slow-motions you should always use the highest frame-rate that your camera has to offer.
3rd party plugins
An alternative to the build-in Timewarp effect is the plugin Twixtor.