Users earn points by receiving upvotes on their posts. Points are based on the type of post. So, for example, if you upload a craft and it receives three upvotes, then you would receive 45 points.

Post Type Points per Upvote
Forum2
Craft15
Sandbox15
Mod25

You can check your points and where they are coming from at anytime by going to View Profile -> Account Settings -> View Points page.