I use plain water in the steam mop reservoir. If you have hard water, use distilled water to help prevent mineral buildup that will clog the steam vents. If my floors are really dirty or a pet has had an accident, I use a bowl of water that has white vinegar added and dip the cleaning pad into it and wring it out to damp and attach it to the mop or use a spray bottle with white vinegar to spot treat. The steam mop has also proved to be great helping to getting rid of fleas. After vacuuming and discarding the bag or emptying the reservoir every time, use the steam mop and do this several times a day with the vinegar treated pad.
Water. Any other liquids that are used in this may cause toxic steam. However, I have had good luck using a mixture of Pinesol and water in a spray bottle over the floor (or other are to be cleaned) then just using water in my Shark. I use the standard cleaner to water ratio as I would if I were mopping the floor.
Just plain water. Steamers aren't meant to have any kind of detergent or cleaning solution used with them. You can add plain white vinegar if you are finding it leaves streaks to the floor. You don't need much. It helps to break up the minerals in the water and leave a streak free clean, as well as, keep you steamer from clogging up with mineral build up.