SESAC is a bit different than ASCAP and BMI when it comes to becoming a member. They are exclusive meaning that you have to be tapped/selected before being accepted into their organization.
When becoming a publisher member you will select a company name and it will be checked through each of the organizations to ensure no one else is currently using the name. Once they have cleared your name you'll have an account set up. There may be a small annual membership fee to be a publisher. It may be something like $50 or $100 per pro. However, double check with each organization to be sure as I'm not 100% on this one.
Remember to handle your other business such as business checking accounts and things. For full details you'll want to download either the Atlanta or Los Angeles Music Industry Connection Books. Now that's Industry Pimping.