Once you have posted your Job, You will be getting proposals from Freelancers. You can talk to any of the freelancer via Message Center and once both of you are agree on certain terms like Budget, Support, Delivery Date and Penalty; You can send an offer to the freelancer.
To do so, go to your Dashboard and click on the Job on which you have recieved the proposals.
To view a particular proposal you can click on the Freelancer Name.
Once an offer is sent to the freelancer, freelancer will take a decision whether to accept or decline or he/she may ask you to modify the offer.