First you will need to transfer your bitcoins from your web wallet, hardware wallet. etc, to an exchange such as Poloniex, Bittrex or Binance. etc,
This example will use Poloniex, but the process is basically the same for the others.
First make an account on Poloniex, choose a strong password, and it's also wise to enable 2FA (2 Factor Authentication).
Click Deposits & Withdrawals
Transferring Bitcoin can take hours sometimes, it will require 2 confirmations on the network to be credited to your exchange account. After that you can dive in and start trading coins, here's a quick picture guide on what's going on in exchanges but there will be a more in depth thread on this.
