1. You go to gridcoin.us and download the wallet and read the steps to get started on linking BOINC with the wallet for rewards on BOINC work.
2. Selecting a project can be a bit tricky. You have to ask yourself why you would chose a project. Do you wish to try to make as many coins as possible, or to join a project with a great cause that you wish to support just because it's that. (Like World Community Grid, Rosetta@Home and other humanitary projects). There are also projects that finds prime numbers and other mathematical problems and then offcourse you have SETI@Home that I think everyone knows about :)

A score for one project doesn't mean you would get the same on another project. It all depends on your hardware and how many that are on the Gridcoin team, as you share the projects reward per project and not in total.

Join the cryptocointalk forum and/or the IRC channel and they can probably give you answers to most of the question you can have.

I wish you best of luck.