So if you visit
https://github.com/sanity/LastCalc/issues?sort=created&direction=desc&state=open you'll see that I've started filing issues. I've used labels to categorize their difficulty, so if you'd like a simple way to get your teeth into the codebase, just pick one of the simple ones.
Please let me know if you are having any difficulty getting the dev environment set up. Just fork the sourcecode and then push your changes when they're done. Obviously please test carefully and write unit tests if possible. I'll try to get people's contributions pushed to the site within an hour or two - so you won't need to wait long to see your contribution go live!