I'm not aware of any license or permits. Investors/Banks will want to know you are really in business, not just some guy with a truck. Open a business bank account. Get liability insurance. Get a bond to cover damages (insurance agent will help you, it's really cheap). Market your services to local banks, the lending department. And local Realtors, they will help put you in contact with investors who buy foreclosures and home owners who may need help cleaning out. Look for Realtors who have foreclosure listings, they usually won't have authority to remove anything, but can help work up the chain to get things cleaned up.
Most of all, go for it! Start the business, push forward, remove roadblocks as they come up. Follow that dream!