Contributions are an important part to every Open Source project. In order to take part of the development of the Mojolicious framework you should fork the original repository on github.
As Mojolicious is a young framework things are changing rapidly. Here is a small workflow which will help you to follow the development of mojo.
git clone http://github.com/<username>/mojo
Basically, that's it - now you have a fresh copy of the repository. You should add the upstream repository to your git repository. This will make merges from the original Mojolicious repository much easier. There is a small section about this inside the github help (http://help.github.com/forking/.
Here is a basic workflow:
git commit -a
git push origin master
After they've pulled the changes you should rebase your repository. This will layer your changes on top of the upstream changes.
git fetch upstream
git rebase upstream/master
git push -f origin master
This cycle will continue to run forever :)
Please note: it's very helpful for the project owner to see pull request with only few commits in them. You should use the rebase command to combine several commits of yourself into a single one before sending a pull request.