Need Help ASAP!!!!.......

Right this is what I know so far:

I have a Renualt Megane 1.4 RT 1998 (S Reg)
Just bought this heap of junk for 2nd car for the missus from the car auction.

Took it to petrol station for fuel, brought it home no probs.
Locked it up.
Following morning I goes out, key fob wont work had to open car manually with key in the door, goes to start the car and immobiliser kicks in???

Engine starts up and dies, what do i do?
Is there a way to bypass this.....

Help please.....need the car going ASAP

Forgot to mention the keyfobs have new batteries in them.