When logged in, go to your Dashboard. At the top of the page, click the "Fulfill 'x' pending rewards" link. The tables below indicate who have claimed each reward. Check the 'fulfilled' checkbox on the right side for each reward you have sent out. You can use this to keep track of who you have sent rewards to, and who you still need to send rewards to.
If it was a digital reward (album download, etc): Email it to your fans. If you need to attach a file, Dropbox and Google Drive are great options.
If it was a physical reward (signed poster, etc): Email your fans and ask for their address. Google Forms is a great way to gather information – you can create a survey and email the link to the reward recipients all at once.
Your fans will love hearing from you and receiving their rewards.