1994 Toyota Tacoma truck wont start

my husband is away- truck has been sitting for about 3 weeks, went to start truck, truck would not start. tried to jump truck off, but only got a clicking noise. light, radio work, but even after several minutes, the truck would only make a repeated clicking noise, like click ,click click, click, - very fast noise. still continued to try to jum truck off, but was afraid of damaging it, so stopped after approx 10 minutes, every time tried to start it, same noise, reapeadted clicking.
i am afraid of getting ripped off by a mechanice, so i'd like to ateast sound intelligent about what might be wrong with it. thank you very much!