If you are over 620, you would need to find an FHA approved lender or a mortgage broker who works with banks that do FHA loans.
Your minimum down payment would be 3.5% of the purchase price. Your closing costs can be paid (up to 6% of the purchase price) by the seller.
If your score is lower than 620, there may be programs available through local housing or action agencies. Many states have them where you can go as low 500, but your rate might be much higher than if you go FHA.
Hope this was helpful.