there is a product most pharmacies like- walgreens, rite aid, cvs etc carry called tecnu there is a wash and an ointment for treatment after it clears it up fast but in the meantime at very least wash off completely with antibacterial soap and lukewarm water Good luck!
Wash with dawn dish soap. It is the oil on the poison ivy that has irritants so if you wash it off quickly it should keep things to a minimum exposure. Then wash the towel and wash cloth in HOT water to keep it from getting back on you. .
Wash with Dial soap and keep itchy area coated with calamine lotion. If the area is truly massive, you may need to go to an ER. They say don't scratch, but it's very hard not to. There are also some ointments out there now that are anti-itch...but I can't remember which ones.