You can flush the freewheel without removing it from the wheel, I just find it more effective when I remove it (but I have all the tools, etc, so removing a freewheel for me takes about 15 seconds). So if you are lacking tools, just remove the wheel from the bike and flush it many times with the freewheel still attached. I have done this on at least 50 bikes in the last year, WD40 has worked fine. You just have to let it dry (evaporate) prior to oiling it. Go to the park tool site for details.
If you are not comfortable removing rear wheel from the bike, I would just take the bike to a shop and have them work on it. Sometimes, that is for the best. Or find a shadetree bike mechanic on Craigs List, they pop up around here from time to time.