Why do you have extremely poor credit? You need to find the root problem before you go on the path of trying to qualify for a home loan purchase. Did you suffer a one time catostrophic medical event that you can document or, do you just have a history of not paying your creditors?
Are you max'd out on credit cards? While that will typically lower your credit, it doesn't typically cause "Extremely poor credit". Regardless, you have to solve the credit issues and change the conditions that caused your credit to be extremely poor before you embark on the path to home ownership.
Home ownership is a tremendous responsibility that impacts not only you but, your neighboors if you decide one day you no longer care to be a homeowner and just walk away.