Don't mess with main breaker if you're not qualified.
Wires on main breaker are Hot even when main is turned off.
The only way to turn off main is to pull meter.
1) There are 2 Hot wires that connect to main breaker.
Each wire is 120Volts, and each 120V line is 'out of phase' with the other.
Two out of phase 120V lines create 240Volt circuit when they are wired together.
Read more on links below.
2) It sounds like your main breaker needs to be replaced, unless total of all circuits exceeds 160 amp. Turn off small circuit breakers and try main breaker again. If main resets, then turn small circuits back on one by one. Your main may still be bad.
http://waterheatertimer.org/How-to-replace-main-circuit-breaker.html3) Here are more links that explain electric wiring.
http://waterheatertimer.org/B220C.htmlhttp://waterheatertimer.org/See-inside-main-breaker-box.htmlAdd a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on breaker or any do-it-yourself project.
Fixya is always less expensive than a service call.