Who tends to have these very self-referential plots - I think you can jump in with Eleven if you like, but it's better if you start with the beginning of New Who - the beginning of the recent revival - and that actually starts with Nine.

Incidentally, Nine was freaking awesome and amazing, and for my money, way better than Ten, so you should start there just for the sheer wonderfulness.

Also, Jack Harkness was introduced during Nine's season (he only lasted for 13 episodes). So that's another good reason to start there.