To get started you need to purchase Bitcoin from sites like Coinmama, BitPanda, Cryptopay etc etc. Assess them for fees and work out your best option. Depending on the buying process, you may need to have an address to send your Bitcoin to in which case you should already have opened a trading account with an Exchange such as Poloniex (the biggest) Kraken, Bittrex and many more. These are centralised exchanges. Or, if you want a more secure, safer trading platform that is decentralised, try OpenLedger aka BitShares. Good luck.