For federal programs you can apply before being accepted by filling out the Free Application for Federal Student Aid (FAFSA). This covers most of the main aid programs including Pell grants, Perkins Loans, and Stafford Loans. You want to apply as early as you can. I think the first day you can apply for the upcoming Fall semester is January 1st. When you fill out the FAFSA it will ask you for colleges you are considering. If you don't know that is fine, go ahead and fill out the FAFSA and have it sent out to schools once you know.
When a school accepts you they should also offer a financial aid package that will list the expected costs and the expected aid they will give you. This information can impact your decision to attend or help choose between different schools.
There are other aid programs such as some scholarships which may be school specific.