If you'd like to work on something that isn't in a current issue, especially if it would be a big change, please open a new issue for discussion!
First, you'll need git to fork and clone the repo. GitHub has help pages about setting up git, and once you've done that, you should be able to clone the repo and change into the repo's directory from your terminal:
git clone https://github.com/{YourFork}/Rg.Plugins.Popup.git
If you are using SourceTree or another kind of visual git application, enter the url to your fork in the add new repository field.
After you are done pull the develop branch, and open Rg.Plugins.Popup.sln in Visual Studio to get started.