First, find your project invites. Visit your "Dashboard" Page and look on the left side, in the box labeled "Invites & Proposals"... the "Project Invites" tabs will list all your current invites.

Click on an invitation you'd like to consider. That will open a page displaying the project's details. You may also have a personal message from the client, which you can open by clicking the blue text that reads "Read Client's Invitation," (found in the bottom-right corner of the "Project Description" box).

To accept, you have to respond with a project proposal. Click the yellow button in the top-right that says "Submit Project Proposal" and fill out the details to let a client know you're interested.