If you've found a deal that nobody is touching, then go ahead and put in a low-ball bid. Sometimes the bank has strict figures they need to reach that'll prevent the sale and sometimes they'll just let it go.
If you don't have enough cash or if you don't know enough about spotting and estimating what needs to be repaired, be sure to have the appropriate contingencies in your offer, even though that may lessen its impact.
(ARV x 70%) - Rehab Costs = Wholesale Price
Some investors are either looking for greater profit or unwilling to take as much risk, so their formula is this:
(ARV x 60%) - Rehab Costs = Wholesale Price
Once you figure out the wholesale price, you can start make offers from there. Make sure that your offer is contingent on a satisfactory home inspection. That way, if the inspector finds out that the house needs more repairs than you are willing to make, they you can get out of the contract and get your earnest money returned.