On the Sell Your Item Form go to Set Shipping and Options and beside that is the link Options.
The Reserve Price is a pop up.
BTW, if your item doesn't sell at the Reserve Price you will still be charged.
If youset your opening price at the Reserve and it doesn't sell you will NOT be charged.
Many buyers backbutton away from Reserve Auctions.
If you do use a Reserve, put the amount of the Reserve in the text of the description or in the title to let potential buyers know if their bid will be in the right ballpark.