Oh ita, I was gonna post earlier about your car, but....
eta: the first thing I'd try is turn the lights on and see how bright they are. If they're dim, then the battery is low. If they're bright, try to start it and see if the battery dims when you do that. If not, then the starter isn't getting power (which could be the solenoid) or the starter might be bad. (A starter solenoid is just a relay - it takes the small amount of current flowing through the ignition switch and uses that to flip a mechanical switch to allow a large amount of current to flow through to the starter.) (end of eta)
Anyway, the clicking sound you hear could be the starter solenoid - this can happen if the battery has only a little charge left. In which case, a jump might get it started. Have you driven it much recently? If so, then the battery might have run down for some reason, like it isn't charging when you drive it. Was the 'Alt' light on (or the alternator gauge in the negative) while you've been driving recently?
Anyway, I'd remove and clean the battery cables and battery terminals and then reattach them (or have a mechanic do that). A small wire brush works good for that. Then jump-start it (if you jump it and you still only get the clicking sound, it might be the solenoid itself) and drive it for a while and see if the battery gets charged while you're driving. If not, then you might need a new battery. Otherwise it might need a new alternator or starter, or something else in the charging circuit is bad.
But there's a good chance it's not something horribly expensive.
Oh, and if you know the battery is old and probably needs to be replaced soon anyway, you could just buy a new battery and stick it in (they come fully charged) and see if that solves your problem. If the new battery won't stay charged then you'll have to look at the alternator or charging circuit as well.