Let me start with a couple of caveats. I don't know what country you're in. I don't know your age. I don't know your background or experience level.
Here are some things that I did (or saw peers do). In HS and college, shoot for the local papers (in college, it's the college paper, in HS it's the local/regional paper...the weekly thing that comes out or a free-press alternative that focuses entirely on local stuff). There are never enough photographers around to cover HS football and basketball games. This is going to do a couple of things:
--make some connections with editors
--establish a track record (that you do indeed show up where assigned, you do indeed produce results)
--create a portfolio (granted, most of it won't be useful for anything other than sports but press officers are used to dealing with local photojournalists who cover everything from the mayor's press conference to the state beauty pageant to the HS football championship and by the way...go over to the police academy and cover the hiring of the new police commissioner).
Apply for credentials. Almost all major police and first responder departments (meaning...at the city level, major counties, and state police departments) will allow you to apply for credentials to cross a police line. Once you've got that (and a point of contact...like a press officer or media affairs director for the local dept.) than it's not that tricky to get a chance to tag along or shoot a training drill or do a story on "a day at the firehouse".
Connect with a freelance writer. If you've got someone who wants to write a story (i.e.: "gun safety tips from the police range master" or "checking your house for fire hazards") that a local fire dept. would want to cooperate on, you're the Jeff to their Mutt (man, I just dated myself there). The journalist gets y'all in the door, you tag along to shoot pictures for the story. Once you've got an "in" with the department than subsequent applications or requests should be easy.