Venus fly traps are very sensitive to chemicals in water. Only water with a TDS (total dissolved solids) measurement of 50ppm or less should be used. Unless you have a TDS (total dissolved solids) meter to verify that your tap water is low enough in TDS, you should only use distilled water, rain water or reverse osmosis water.
To actually water the plant, you can water them from the top. Most growers just put the plant's pot in a tray of standing water. The medium that Venus Fly Traps grow in sucks up the water like a sponge. You can see a picture and get more information about watering your venus fly traps on the watering Venus Fly Traps page.