Hey there!
It's great to hear that you're interested in writing for the site! Make sure to create a sandbox page for yourself, and get any drafts you write up there shown to the IRC chat and/or the drafts forums for feedback prior to a mainsite post.
The chat is also a great place to hang out at and meet fellow Foundation personnel. You can get a feel for the real-time critique given there, and also toss around ideas with others. The chat is pretty laid-back, so you might find all sorts of topics being discussed there.
Regarding writing, if you'd like examples of what makes a successful SCP nowadays, consider taking a look at the list of top rated pages of this month, and the accompanying archives if you'd like to look back in time a bit. It's of note that while there are many inspirational articles in Series I, articles written four 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.
I also recommend reading some tales to get a feel for the worldbuilding we've got on the site. The In His Own Image series in particular is an excellent introduction to the Foundation 'verse.
Furthermore, if you've got any questions, comments, or concerns, feel free to PM me or another staff member.
Welcome to the site!