Welcome Chess, great to have you onboard! (no pun intended)
It seems like you have the right attitude so far, just take some time to see everything possible on this site, learn and grow, and then contribute back to it!
I would suggest starting off by reading some of the featured storygames, and seeing exactly what it is that readers enjoy and expect on this site. Which is to say a story with plenty of choices, intrigue, plot, characters, arcs, and structure! No need to worry if you're writing is not perfect, no one's is (with the exception of maybe 2-3 writers on this site), no is going to rail on you for bad writing or writing that is not necessarily the best.
Most members are happy to give you feedback and criticism, as well as help you develop your skills in writing. I would suggest start off with a shorter story (maybe in the 6k-10k range) then as you get the hang of cys' editor, move on to take everything you've learned and make a larger story.
With the coauthor aspect, there are plenty of authors that would be happy to do that. For right now though, most people are busy writing their own stories for the contest and otherwise. While it would be hard to find someone to coauthor with, it is still possible. I would suggest getting a story or two out first, to show you are reliable and eager to write! No one wants to coauthor with someone who they think might expect them to do most of the work in writing, editing, and such only for them to abandon the story and never come back.
In the meantime, while you are working on your story and writing skills, it is a good idea to read, rate, and review storygames in the meantime. Authors always appreciate the ratings and feedback.
Another thing when it comes to criticisms and feedback, even if you don't exactly like what you hear, it is usually a good idea to thank whoever is giving you feedback and remain calm. If you really have to disagree on a point of feedback (if it's a style difference or something essential to the story) you can always politely do so. Pretty much every member that I have seen the members have an issue with, usually respond atrociously to feedback. If you flail and act like a dumbass, everyone can and will obliterate you. Members will appreciate seeing you make efforts to improve your writing!
Not sure if that is everything I wanted to cover or not. If you have any questions always feel free to PM me.
Edit: Just noticed you have been on the site for a couple of years already. Better late than never for one of these posts and trying to get more involved I suppose!
