If we are discussing the request you received in your inbox, then click on "Create an Offer" and generate a customized offer. It is up to the client to decide whether to accept it or not. If the client has created a Brief, the procedure remains the same: create an offer. However, in this case, you might not be the only one submitting an offer, so the chances of the client accepting your offer are lower.
If you have been notified about the brief but cannot locate it, click on the bell icon in the top-right corner of the page, next to your profile picture. There, you can find the necessary notification.