First - learn to recognize poison ivy, poison oak, and stinging nettles. All 3 will give you one hell of an itch! But, since you've already run into something, try this: wet the affected area with COLD water, pour rubbing alcohol over it, and rinse with COLD water. Hot or warm water will open you skin pores, allowing whatever it is to get deeper into your skin. There are a number of poison plant treatments on the market for washing stuff like this off - ask your local pharmacist which one is the best, and follow the directions carefully. Meanwhile, take an allergy pill - OTC stuff like bendryl might ease you itch!
Whichever plant you ran into has an oil which you are allergic to. So, getting the oil off of you is the first step (which the rubbing alcohol will do), and treating the allergic reaction is the 2nd step.
Hope you feel better!