A really short introduction, just to get you started.
Create a project from scratch based on a BitBucket template
Set up Mercurial on your machine if you haven't already.
Code:
mkdir /path/to/your/project
cd /path/to/your/project
hg clone https://user@bitbucket.org/user/project-template
Create your first file, commit, and push
Code:
echo "user NA" >> contributors.txt
hg add
...