Hi Penny-
How much to bid is really up to you. Have you verified the price for the dress new? What is it worth to you? Only you can really answer that.
In terms of bidding, if I am gong to be around, I like to watch and bid at the last minute. That way it doesn't give anyone else who bid or wants to bid time to think about my bid and decide to try a little higher. The thing with bidding at the last second is, you really have to bid the maximum you are willing to pay for the item. I have my system down where once my bid is confirmed, there are 4-5 seconds left in the auction. Not enough time to put in a new bid if I am outbid and decide I want to try again.
If you have never bid before, take the time now to get yourself registered with eBay so you can bid. On the eBay homepage, under the boxes that say BUY, SELL, My eBay, COMMUNITY and HELP, there is a link to sign in or register. Click on register and follow the steps. Then make sure you are signed in.
Hope I helped. Good luck. It's a very pretty dress.