Example: Person A and person B are applying for a joint mortgage. Person A has credit scores of 800, 790, and 750. Person B has credit scores of 700, 615, and 590. Then the JOINT credit score will be 615.
The next thing you need to think about is your debt-to-income ratio (DTI). What will your total monthly debt (mortgage payment, property taxes, homeowners insurance, private mortgage insurance, credit card debt, car payment, student loans, personal loans) be compared to your gross (before taxes) monthly income. Different programs have different guidelines, but for FHA the max DTI is 43%.
With all FHA loans you must pay private mortgage insurance (PMI) for the life of the loan as well as an upfront premium of 1.75% of the loan amount. You are allowed to finance the upfront premium (that means add it onto you mortgage) but that means that your loan-to-value (LTV) will be 98.25%(100% purchase price - 3.5% downpayment + 1.75% PMI premium). Then for loans less than $625,500, you pay a 1.30% premium per year. The only way to get rid of PMI on an FHA loan is to refinance once the value of the mortgage is less than 80% of the value of the home.
There are also closing costs, also called finance charges. A general rule is to assume that closing costs will be about 4% of the purchase price, but it's different state by state. If you don't qualify for any kind of down-payment or closing costs assistance, then you'll have to pay that as well.
As far as down payment assistance programs, you need to research what is in you area. A great place to start is Neighborhood Housing Services of Milwaukee. Set up an appointment with them to see what you are eligible for. They can help you apply for grants, improve your credit, and find homes that are in your budget. There are other non-profits that can help as well, but you'll have to do your own research there, as I'm not sure what is in your area.
I hope this answered your question!