Here how to do it: Move along at a moderate speed as if you were going to ollie, with your feet in the same position as if you were going to ollie. Snap down on your tail, and slide your front foot forward towards the nose. While doing this, twist you body frontside until you turn a complete 180, stay centered above your board, and land on the trucks. If you landed correctly, you should be riding fakie, or backwards.
Many people have trouble learning this trick. Common problems are people who can't turn the complete 180 and people who can't land with both feet on the deck. If you're having trouble, watch people who can frontside 180, and you will learn it eventually.
Another type of 180 ollie is the backside 180 ollie, which is slightly harder but still definitely worth learning.