How can I start the auction
I assume you mean bid in the auction.
FIRST: BE AWARE. Auctions are all geared toward the SELLER. We, the buyers, pay all the auction fees. Right now most auctions are running 15%-20% fee above the gavel price. So you bid $800 what you think is a steel for a .500Mag, but you'll end up paying $960 PLUS whatever the shipping and FFL fees. You have no control over those fees either.
Three ways, with Rock Island Auction.
1) Go to the auction yourself. Register and obtain a number, then participate in the auction, live and in real time.
2) Go to RIA web site, register, then place Pre-Bids for the items you want. You basically set a beginning bid and a max bid. On the day of the auction they include your bid and will increment it until you reach your max. There's no real time opportunity to change your bid once the auction starts.
3) In between these options is Proxibid.com There you can participate live, via Proxibid, and actually bid in real time. Trouble is these auctions move very fast and there's no guarantee the Proxibid agents on site will get your bid place before it closes.