tino immobiliser

hi,new to forum just brought a 2001 tino decided to put a after market stereo unit in did'nt disconnect battery so everything was ok until i did a job on the engine so had to disconnect battery so now the immobiliser has stopped the car starting have been informed that i can disconnect it so this never happens again all i can explain is i have the black key with lock & unlock and i think the immobiliser is clipped on the ignition barrel so if anybody couyld explain how to bypass the unit so it never happens again i would be very thankful,many thanks.