It seems natural to have one git repository per VS solution. For one thing, it's much easier to remove an old deprecated solution if it has its own repo. But git supports both: you can have one big repository with multiple solutions too.
This site is worth doing as an introduction. For much more detailed information, you can download the Pro Git book.
Learning git is not easy, and using git is definitely more complex than using Visual SourceSafe. But it's very much worth it.