In your account homepage, click on the Jobs button on the top navigation bar. You will be able to see a list of jobs and projects currently available. You can search and filter projects or jobs by categories, required skill-sets, location and etc.
You can click on any of the Jobs or Projects on the list and view them in detail. Once you have found a job that you are interested in, you can send your proposal to the Hirer by filling in the required fields in the form and clicking on the Send Proposal button at the bottom of the page.
Make sure you've completed your profile with your experiences and skill-sets. A completed profile will get you noticed by the Hirer.