Speak to the financial aid department, before you do anything else. Each school has a financial aid department, and they have regulations about whether or not you can register before your loans come through. Some schools will let you, but others will make you wait until you know your financial aid status.
Fill out your FAFSA and other financial aid paperwork -- as required by your school -- as soon as you know you want to attend classes. When you are finished filling it out, submit it right away. You should get a preliminary answer quickly.
Go to registration -- or to your academic advisor -- when it is time to enroll in courses for the semester, or the quarter. Tell them that your financial aid status isn't known yet, but that you have filled out paperwork. The person in charge will look up your financial aid paperwork on the computer, and will be able to tell whether or not you will get the loans. Usually, if they can see that you will get loans, even if those loans aren't certified, they will let you enroll in classes.