Re: hello all new here with a few questions :)
If it is a cruiser it won't take a disc brake. You will have to convert the front end if possible to a mountain bike front end. The front hub will have six threaded holes on the left side for a rotor to mount and the fork will have a welded tab on the left fork for a caliper to mount. You will have to find a mountain bike fork that will work on that frame. I think most cruiser frames use a threaded headset. You can find a threaded mountain bike fork or change the headset to a non threaded since most mountain bike forks are non threaded. If you don't know much about this stuff you can take the bike to the bike shop and they will tell you if it is possible.
If you don't want to do all of that and spend that much money you should first look to see if the fork and rim you have now will accept a rim brake. If so that should be ok. Not as good as disc but still pretty good. The fork will have two posts sticking out and the rim will have a flat side wall for the pads to rub against.