Hi, Seren, and welcome to WriteWords
Yes, it is a confusing site - took me a few weeks to find my way round - but if you get stuck just give us a shout and we'll be happy to help.
As a trial member you can join one Group and upload one piece of work (once it's uploaded you can't edit the work so choose carefully, something around 2000 words is the best length).
Here is a link to the groups (You'll see another link at the top of the page, where it says Archive, Groups, etc):
http://www.writewords.org.uk/groups/
Look for one of the more active Groups on the list, and Click to join. There'll be a few hours to wait while your request is pending, but then you'll be able to upload an extract or short story. Alternatively you can upload a piece of work in the Archive - but work tends to get overlooked in there.
- NaomiM