Coinbase is probably the easiest to start with. You lose some of the REAL crypto/anonymity from BT, but it's very easy to buy/sell using your bank account.
If you can use paypal, you can use coinbase.
In fact, my 12 year old son uses it. I think I've given him $5USD twice for doing weekly chores and he's already made 15%.
It's been a good teaching tool. Learning about dollar cost averaging, currency exchange and other financial concepts.