Make sure to allow NuGet Package Restore in VS (Tools > Options > Package Manager). The first buid of the project will take quite a while, be patient, it will finish at some point.
Upon build a web.config file will be copied into the `OurUmbraco.Site` project which you can use in the following step.
If you're working on the frontend (the js/css/etc parts in `~/OurUmbraco.Client`) then you can either run `~/build/BuildClientFiles.bat` to build them and have them copied into the site or `~/build/RunGulp.bat` if you're actively working (gulp will monitor changes, build and copy). Or if you have npm/gulp installed on your machine you can run the usual commands in the `~/OurUmbraco.Client` folder:
Restore the database to SQL Server 2012 SP2 (won't work on earlier version) and update the connection strings (`umbracoDbDSN`) in `OurUmbraco.Site/web.config`.
To log in, try `root` / `Not_A_Real_Password` for the backoffice and `member423@non-existing-mail-provider.none` / `Not_A_Real_Password` for the frontend.
If the documentation area seems empty then that's because you need to download the documentation, look for the the `Github Documentation` tab in the dashboards in the Developer section of Umbraco and use the `Get Docs!!` button.
If you're creating a pull request, make sure that it's backed by an issue on the tracker: http://issues.umbraco.org/issues?q=project%3A+our.umbraco.org
Mention the issue number in your pull request so we can merge it in more easily.
Even if you're not planning on sending a pull request, you can always create an issue on the tracker if it doesn't exist yet, it helps other find ways to contribute.