Take the punctured tube out, find the hole, match it to the tire, and pull whatever caused the puncture out of the tire. Instead of a new tube, just put one of those valves from the little bag in. Then pump the tire up. If it holds air even for a few minutes, you are in good shape. Let the air out, dump in about 100 ml of sealant, pump the tire back up, and then take the wheel and spin it around both vertically and horizontally until you coat the entire inner surface with the sealant. It should then hoild air like the rear tire.
If the tire doesn't seat on the rim and hold air for a few minutes at least, don't put the sealant in. Take everything to a bike shop, and have them do it with a compressor. Watch what they do. Pay them the $20 and consider it money well spent.