First you will have to check into the rehab and that will involve a bit of red tape just like anything else. It won't take but an hour or two and then you will be checked in and they will put you in detox and probably let you sleep for a few days. Before you do this though they will likely check you out from a medical standpoint and also search your belongings to make sure you are not bringing any drugs into the facility. It is also likely that they will feed you as well, and most places will let you smoke when you want--though that will vary by facility.
While you are going through detox you will likely have withdrawal symptoms. Some of these might get a bit uncomfortable and most rehabs will give you medication to help with those. They will not allow you to get hooked on the medication they are giving you, instead they will give you just enough to feel better and possibly taper you off the dose if they have to.
After a few days of detox they will likely send you to the residential unit so that you can attend some groups and some lectures and group therapy. There you can learn more about how to live clean and sober once you leave rehab, so this is an important part of your treatment process. If you want to stay clean in the long run then you should pay close attention to what they are trying to teach you.
Learn more about heroin rehab and also about rehab Los Angeles.