So you want to speak Leet?
It's simple really. The underlying theme is this: use a number in the place of a letter whenever you can.
For example, the picture on the left uses a zero in place of an "o".
Why would I want to?
Because it's cool. It's like a secret language between your friends and you.
Once you speak Leet you can sit in your tree house and yell "n0 g1rl5 4ll0w3d!!" (No girls allowed.) if you want.