Buy on Coinbase ( if you want to avoid fees look into my blog at @keep0n.steemin and follow me if you like ) , on https://coinmarketcap.com/
you have all of the coins and under each coin go to markets and it will tell you where that coin is traded, then make an account on that exchange and send your btc, once your btc arrives you can buy your coin , good luck.
RE: What platform do you use to trade cryptocurrency?