1999 BMW 323

I've just purchased my bmw from an auction. It's come with only one key and the remote part does not work so can only be used manually. Is it likely to be just the little battery that needs replacing? If so, I've been told by a motor spares shop that if I put it in incorrectly then I will immobilise the car! As I only have one key I'm reluctant to try this. Will I have to purchase a new one from a main dealer (at considerable cost I suspect) or do you know an alternative?
P.S. I have no user manual or any service history etc. for this car

Replace the battery and initialize the key and see if it works! ♥