https://www.coinbase.com is great for simplicity. Diversifying with BTC/ETH/LTC is a good strategy to minimize risk.
You can use my referral link to earn extra BTC when you purchase :D https://www.coinbase.com/join/5927caaa2cc81a08f16dab25
https://www.gdax.com/ is a more advanced trading platform (same company as Coinbase). Expect lower fees in exchange for managing your own trades