Your credit. Credit requirements are tougher now than before the housing bubble burst. In 2005 I could get almost anyone a loan with 0 down, now there are few programs with low downpayment requirements and they do have minimum credit standards.
To repair your credit start by paying your bills on time or early. Set aside money every month for savings, so you don't find yourself needing to use credit. Pay off all your credit accounts starting with the smallest one first, then the next smallest adding additional amounts as you go until you are debt free. Do not close any accounts or pay off old collections yet. Closing accounts reduces your credit score, hurting your goal. Paying off old collections, before your lender requests it, can also reduce your scores. When you have scores in the low to mid 600 range, apply for a mortgage loan. Approval may be conditioned upon paying off the collections then, but you'll already have your approval in hand.
These are just a few ways of improving your credit, search out more. If someone promises something too good to be true in credit repair, be very skeptical. There are frauds out there. Best of luck.