please help

hey guys just bought wifey a 2003 x5 after reading here about remote key not working im kinda stuck
i got only one key and it dont work tried doing the sync but with no success
i can unlock and lock from the door lock and my center locks work and so does the interior light
but i read here so many things can be wrong and im confused can someone tell me if its just the remote battery or the fuse or some box under the dashboard do i have to put my key in the ignition and disconnect the battery


thanks