If you are borrowing the money to buy, you would need to get a pre approval letter from a lender. If you're paying cash, you would need proof of funds. An offer won't be accepted without either a pre approval letter or proof of funds.
If you're buying homes at foreclosure auctions, you often need to have cash and have the ability to pay the sheriff within 24 hours of the auction.
You will also want to find a real estate agent that is familiar with negotiating on and writing contracts on bank owned properties. Most agents have had at least minimal experience doing so, but it's helpful to have an agent that has done at least a couple of foreclosure transactions.