I am new to this too, but thought I'd share what I know...
It depends what country you are from. I am from the UK, and coinbase was the easiest way to get some money into Bitcoin, Eth and LTC, but with fees as I first used my debit card. I would not recommend that in the long run due to fees, but its a good way to get started and play around.
As most exchanges dont accept GBP, but deal with Euros or USD, I had to use Revolut, a banking app as a go between to transfer larger amounts into my coinbase account. There are guides on how to do that.
GDAX is owned by Coinbase and you can use the same login details. The difference is that they have few or no fees. So if you want to buy the major coins, transfer your money instantly from Coinbase to Gdax from within GDAX and buy from there. Also use GDAX to move BTC etc. to your wallet or to other exchanges.
If you want to trade in other Alt coins, then you need different exchanges. I have signed up to Binance which is the becoming very popular. Bittrex is one of the most popular but they arent taking new signups now because there are too many people trying!
Make sure you enable 2fa security on all your accounts including the email you used to sign up. Also make sure you back up the QR codes when enabling 2fa...at the point where you are meat to open Google authenticator and scan the code, print it or take a screenshot. This way if you lose your phone or it dies you can get back in. Without 2fa you are at serious risk.
Thats as much as I can tell you for now