Coinbase. If you haven't used it before, before you can spend any serious amount there's cool off times associated with account verification, so get as much of the verification out of the way as possible. Once you get the max level, they still only let you buy a certain amount, but that increases over time and as you purchase more and more.

From there, you should start taking your position by purchasing as much as possible OR...if you can wait...sign into GDAX using your Coinbase account credentials (they're the same company). GDAX allows you to deposit funds into the GDAX USD wallet, and from there you can do market trades and avoid the hefty (and I mean hefty) Coinbase service fees.

But that all takes time, since the wire from your account to Coinbase/GDAX can take like 7-10 days depending on your bank and Coinbase.

THIS IS A VERY GENERAL APPROACH. You should look into guides about how to avoid service fees using GDAX. There are plenty of guides out there.

Good luck! Be careful when you're starting to build your portfolio to...take the time to research and stay informed.