You can gain EXP many ways, as others have already said. Post submissions, but don't just post a whole lot to get EXP. Take your time, and make good quality uploads.
If you are blogging, make sure you use good grammar and spelling. If you are making a project, make sure you have good screenshots. Best of luck, and welcome to PMC!

Post submissions (projects, skins, texture packs, servers, mods, and blogs). Views, comments, and diamonds received by your submissions earns you experience. Posting in the forum or the chat room does not earn you any experience.

http://www.planetminecraft.com/members/levels/

Before you post a submission, make sure you've read the section of the rules for it via our Rules tab.

phatpig321I have no idea how I get XP! Could someone tell me how?

You post blogs, skins, texture packs, servers, mods, or maps.

You post submissions on the site part. I see people getting the most from texture packs and mods.