Sheriff auctions are posted online and held monthly, depending on the reason for it being up for auction, be it back property taxes or perhaps a foreclosure property you need to look it up on county auditors website to find out who holds the morgage, the bank will ALWAYS bit up to full morgage value for a property at sheriff auction. DO NOT BID AGAINST BANK.
In a Sheriff auction the money's collected are then paid to lien holders such as morgage etc so they get all their money back in about 30 days. It is wiser to go, see if bank is bidding and then wait for bank to list the home. For example the house next door to me, I bid up to 14000 and the bank went to 96000 immediately. They paid for the house to be cleaned out, the yard to be cleaned up then I bought the house from bank four months later for $10050.00. Be patient do your homework, but it is crazy to bid against the bank as they get there money back once house it paid for. You will need a cashier check for the down payment usually 10% to have your bid accepted then have 30 days to pay balance. If this is a back tax issue the current owner of property can make good on debt within that 30 days. If they do pay their obligation your purchase will be voided and you will recieve a refund on your down payment within 30 to 90 days depending on county you live in. So research, got to auction, and if a bank is bidding, walk away and watch for listing, then bid low. You may have to wait a few months longer but you can usually still get the home you wanted and for a very reasonable price. GOOD LUCK!!!!
If it just so happens to be listed at http://www.homepath.com/ then you can use Fannie Mae HomePath financing and put as little as 10% down with a 660 score, but if it's not, then with regular Fannie Mae & Freddie Mac conforming loan programs you'd be looking at a 20% down payment, with 25% down getting better financing terms.
For those options you need to apply & get pre-approved with a mortgage broker or lender.
As far as purchasing the home before it goes to auction, is it listed on the market currently? Is the current owner selling it as a short sale?
If you are trying to purchase it at the auction, in some states you need to have your financing squared away within an extremely short period of time after your winning bid is accepted, as in 24-48 hours, and that wouldn't allow any type of traditional financing to be used, and you would have to either buy with cash or use private/hard money with a lender who is ready to fund in that period of time (the trade off is the interest rates are in the teens and the fees can be nearly 10% of the loan amount).