I've been reading SCP for about 4 years now and have only just now decided to join the site, I don't really have a reason for doing so, I guess I was just nervous or something (I know lame excuse). I orginally found out about this site when SCP - Containment Breach was made and started gaining popuarity, since then reading SCP has easily become my favorite thing to do in my spare time. I think I've read through about 2/3 of Series 1, 1/4 of Series 2, and a bit of Series 3. Anyways, I'm just glad I finally convinced myself to join and look forward to contributing to the lore that has kept me hooked for so many years.
Hello there! Wow, you've been reading the site for awhile. Good to have you here!
So far as contributing goes, I do recommend you read more Series III and II—There are many inspirational articles in Series I, but articles written six or more years ago are less likely to be up to current community expectations, even if they've got a lot of upvotes. Remember to pay attention to how old an article is when deciding if you'll use it as a guide or not; you can (and should!) check article age by clicking the "History" link at the bottom of a page, and looking for the page creation date.
In regards to your own writing, you should first create a sandbox page to keep your in-progress works, references, or ideas in development at. When you've got a draft ready to be read, bring it to the IRC chat and/or the drafts forums for reviews so you can make the necessary edits before a mainsite post. Also, do poke around the Guide Hub and the other writing help forums (General Advice for questions about the writing process, Ideas and Brainstorming for polishing up concepts) if you're unsure about anything.
I look forward to seeing what you come up with. Welcome to the site!