If the alarm system is aftermarket the fun has yet to begin!
If it's M.B. and someone has messed with it you're still in for a version of the Marx Brothers automotive experience.


Look under the hood , there may be an alarm horn mounted on one of the wheel
wells.(kinda looks like a "BullHorn") detach the wires leading to it.[at least until
you find out what's wrong with it.]

Unless you've got the HooDoo Hex on your electrical system , the starter's
trying to go to that geat Bosch recycling facility in the sky.(Assuming the
connections and cabling to the starter are in good order)

Try putting a "Jump Start" on the system with the new battery fully charged.
Positive cable on the battery(s) positive posts...Negative cable on the recipient's engine block and then to the donor's engine block.
If you can reach the "Bendix" on the starter (looks like a small cylinder on top
of the starter) tap on it gently with a small hammer whilst someone else works
the key.

Observe the MOST stringent safety precautions when attempting the last suggestion!