You will need a pre-approval from a established lender. Sometimes a "no-name" broker pre-approval letter will not fly. Also, confirm with the agent that there is not a dictated lender to pre-approve with. Some will offer free appraisal and loan aps if the pre-approval is through their desired lender. Either way, a pre-approval is essential.
Have your agent run the comps of last 3 months within one mile of listing you are interested in. Also look at active listings and compare with a walk through. Know the value and know what you are willing to spend. Have your agent or other professionals help you estimate any repairs needed (if any) to move in. (Paint, sod, appliances...)
The cleaner the offer the better. Expect to hear " your offer has been countered and the owner would like to see your best and final offer." if your offer is accepted, be clear on contingency periods (they are passive releases with many REOs). Above all, read everything very carefully.