The first thing you should do is contact a mortage lender/broker. You could go to a bank if you choose, but with a mortage broker deals with many lenders and your chance of securing the type of loan that best meets your needs is much greater. You'll meet with them and give them all your financial information, income (a good idea is to have your last 2 pay stubs), list of assets, list of creditors, the balances you owe your creditors and your monthly payments. They'll run your credit reports and advise you what the range of the loan you could qualify for based on the current rates. They'll also advise you if there's anything you need to clean up on your credit report. Since the banking colapse, they may require much more documentation at this point then they did years ago.
I highly recommend you get your free annual credit report before you meet with them. Review the report to make sure it's accurate.
I can't stress this enough..... make sure you read the fine print and fully understand what their talking about. Take some time to read some of the articles in the money sections of msn.com or aol.com and become familiar with the terminology.
Remember that the rates change daily, so from the time you start the process to the time you secure the loan the rates will change. Also, they are just "pre-qualifying" you. There is no guarentee that you will get the loan.
After you have the pre-qualifying amount, start house hunting. Once you find the house you want to purchase, you will then officially apply for the loan.
Good luck.