Coinbase is "the PayPal" of bitcoin. Meaning, it's the big, well-known, professional financial service company. Fees are cheap (1.5%) and smaller amounts can be transferred to your wallet instantly. They accept credit/debit cards too (at a 4% fee). The Coinbase wallet also has bitcoin insurance so you are protected from hacking and loss for up to $250,000. They also have a pretty decent app that doubles as a wallet.
Downfalls: they put a lot of limits to buying. Buying limits are often very low, and they are are notorious for long holds in buying bitcoin (3-5 days). Reversals happen a lot when the price changes too much. Also, there is no privacy with this option. If privacy is an issue, stay away.
CoinATMRadar has a map that can locate a bitcoin ATM near you. You can get your bitcoin immediately, up to a few thousand usually. It's more expensive though. A 8% - 12% fee is common. Just choose the cheapest one closest to you.
You are "buying bitcoin on the street" with this option. The bitcoin industry is notorious for scammers, so be careful. The best strategy against scams is to choose the dealer that has the longest and best track record. Length of track record is usually the best predictor of good, honest behavior. This an expensive option (around 10% fee), but the advantage is that the transaction can be done discreetly.
Some dealers allow bank deposits, which can be convenient. But again, please make sure the dealer has a good track record before you do business with him.
1) Use Coinbase if you need ethereum quickly. Exchanges such as Kraken or Poloniex are other alternatives as well, but I consider Coinbase easier.
2) If you already have bitcoin and need convert to another crypto-currency, such as ethereum, use Shapeshift. Shapeshift can convert bitcoin to any crypto-currency you like (ethereum, litecoin, dogecoin, whatever). You don't need to sign up for an account either, and the conversion can be done immediately. ShapeShift has a pretty competent and speedy technical support team from my experience, just make sure you record the transaction id of every transaction with this service.
The Bitcoin Spot
5152 Lankershim Bl
North Hollywood, CA 91601
Hours: Monday - Friday 9:00AM - 5:00PM