Check to see if the leak is coming out of the stem with soapy water as suggested, and if it is tighten the valve core and check with soap again.
If it still leaks get the core removal tool and the new core handy, if the tire is still inflated you can replace the core easily, unscrew the old core as fast as you can and quickly insert the new stem without it getting blown out of your grip. If you are quick you only lose about 10 to 20 PSI, if you are slow you lose it all. Re-inflate and re-test with soap, good luck.
Some valve caps come with the slot in the top that are required to remove cores, these are steel caps and are good to keep around if you have one.