First, you have to buy some bitcoins, obviously, if you are going to trade them. Then you are going to chose your trading pair, like for example BTC/USD. Next you are going to choose an exchange that has the chosen trading pair. When you sign up with that exchange, you will be given an address to make a bitcoin deposit. Then you will transfer the bitcoins you bought in the beginning to that address on the exhange. Once the funds are visible in the control pannel on the exhange, you are free to start trading.