Hi Kate and welcome to WriteWords
It's usually best to join one of the groups and upload the piece there, as it is liable to be overlooked in the Archive. We have a very active Childrens Group which extends into Young Adult if you fancy checking it out (there is an old YA Group but it's pretty much mothballed these days).
As a Full Member you can move around the groups and join more than one group while you find one you're comfortable in.
The Site does work best with reciprocal critiquing, so please don't be afraid to chip in.
Looking forward to reading your upload - especially on such a wet & windy afternoon, today.
- NaomiM
<Added>Just to add, if you need any help navigating the site feel free to ask - it can be a little confusing at first.
Groups are here:
http://www.writewords.org.uk/groups/