2002 Ford Focus

You can do a quick check with a voltmeter:
check voltage at battery; you should get 12.6V.
If you have lower than that, your battery is probably not strong enough to crank your starter. If you don't have a voltmeter, get a friend to drive you to an autopart store where they can check your battery for free.
Average battery life is 4-5 years. If you never changed the battery before, that means your current battery is 6-7 year old. You may need a new one.
If your battery tests okay, come back and we'll troubleshoot it from there.