Hello there.
I'm going to go a bit more in detail on the first response.
1. Yes, critique here can sometimes be harsh. The site has very high standards, and there are no exceptions. We expect quality work, especially given how many resources are available here. Make a sandbox page for yourself, and get feedback from the IRC chat and/or the drafts forums before you post to the mainsite. Read through the guides in the Guide Hub and use the information there to go through your drafts to make sure you've got reasonable containment, interesting expungment if any, etc. A lot of questions about how to write for the site are answered well in the guides.
2. While it's true that you're free to keep whatever headcanon you want, keep in mind there's a few things that are well-established site-wide (e.g. no more testing 682 on everything, no Foundation scientist has fifteen PhDs, and so on). We also have a Canons section which details different views on the Foundation 'verse itself. What it comes down to though, is that if you write it well and people like it, then it stays on the site, no matter how crazy it is.
3. Even if you don't plan to write anything, you can still vote, comment, edit (grammar and spelling only please, and if you think something should be fixed but aren't sure, feel free to make a post and/or message a staff member), and make forum threads.
Also, if you're looking for SCPs that are a good example of current site standards, I recommend reading the high-rated Series II and III articles (SCPs with numbers from 1000 to 2999). There are many inspirational articles in Series I, but 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.
If you have any further questions, of course, feel free to ask them. Welcome to the site!