Why doesnt my car start and the car alarm keep sounding after I replaced my 2002 RX300 battery?

With the doors shut, insert the key into the drivers door lock. Lock and unlock the door 5 times. This should reset the system. If that doesn't work you should contact a qualified repair shop

I tried locking and unlocking the door 5 times and the problem still persists.

Then you will have to call a locksmith to reset the anti theft system. He will come to you. The only other option is tow it to the dealer.

Thanks for your help.

I don't think this is an interference engine. Which means if it breaks it doesn't trash the engine. Have a belt put on and it should start up.