2002 Ford Focus SE wont start

Since this summer my car has had a habit of shaking and jerking when idle, to the point that it shuts down, so I have to turn it off and start it again. It does this at stop lights or while waiting a few minutes at the stop sign. Yesterday I had to jump start my car, it worked wonderfully, today I had to run errands before work and it worked just fine, well when I had to run an errand for work, in which I had to run to the bank and go through the outdoor teller line. I had to wait for two other cars ahead of me, so to keep from the jerking and shutting off that it does I just turned off my car and waited, it would start up fine and when it was my turn and I had been helped I tried to start it up my car. It wouldn't start just rapid clicking, and when a friend came to help me jump start my car it wouldn't. So I had to leave it at the bank after pushing it our of the way. I am a college student so I don't want to spend money what little funds I have on something that wont fix my car. Can you help?