Once you have successfully registered, you can use the portal depending upon the account type:
Freelancer (Seller) : You will see many jobs listed, find a relevant job and apply on it. Your application will be sent to the Client (Buyer) who have posted the Job. Your client may ask questions through message center and he may hire you thereafter.
Client (Buyer) : You can post your jobs as per your requirement and will get applications from freelancers on it. You can filter the applications, interview the freelancers through message center and then may hire a freelancer.