Well when you design a List for the item it should have an option and you select it by clicking change:
If you cannot see the option to set a reserve price you click the 'Add or remove Options' and then tick the box that says 'reserve price' in the pop up.
There is also the fixed price Tab you can select.
I'm not sure whether Ebay allow you to change this once you've already put the listing however.
Also under certain categories the buy it now option is disabled I think