To pledge to a project, make sure you're first logged in to your Kickstarter account. Once you've found a reward that interests you, just click the green 'Back This Project' button on the campaign page. Alternatively, you can scroll down to select a reward tier.
After you have selected your reward tier, you'll be prompted to enter in your payment information. If your reward involves shipping, be sure to select the appropriate country from the drop-down.
Lastly click the "Pledge" button and you'll be all set! You should receive a confirmation email after completing your pledge.