The main factor for a first time home buyer securing the best interest rate is your credit score. Higher the credit score, lower the interest rate (regardless of down payment). In order to obtain the lowest market interest rates, it is ideal to have a credit score of 720 or above. That being said, that is not a requirement. You can still have a credit score in the 600 range and obtain a slightly higher interest rate. A buyer that I am currently representing just locked in at 4% with a mid range credit score of 665.
You must also have steady income and low debt to income ratios. While you must have steady income, the income amount has more to do with the amount of the loan rather then the interest rates.
Out of all of the Buyers that I represent, first time home buyers are by far my favorites! Here is some information on my website I published for first time home buyers: http://www.homeskatytx.com/informationandfaqsforfirsttimehom
Hopefully the information provided in this link will answer a few of your questions. Don't hesitate to call me at (832) 594-1265 or email me at firstname.lastname@example.org if you have any further questions or need recommendations for a lender.
Best of luck!