Coinbase seems to be the simpliest option. Fees are reasonable and the wallet is pretty good. That being said why so little, $20 might grow to $50 or even $100 if your really lucky over the next couple years, but that's nothing in terms of actual USD profit, I would suggest trying to find at least $50 at minimum ideally around $100 if you want to see any substantial profit. But if you just want to learn how the system works and aren't serious about investing then you can go ahead and just buy it on coinbase,