I forgot to put a reserve on an item, how can I fix that?

As long as the listing has no bids you can revise it to have the reserve price.  If a listing already has bids you would have to cancel the bids and end the listing.  eBay will charge you for doing this if you have already done this once this year.  Then you would have to relist the item with the reserve price.

But why pay at least $2 for the reserve price?  Many buyers do not like reserve prices and will pass your item by.  You should always start the auction at the minimum you are willing to take for the item.