Yeah, I'm not quite clear what you mean, either.
Are you talking about the press screenings that are done in advance of the release? If so, there are two ways to get tickets to those.
1. Have press credentials. Each studio has a local person in charge of promoting these events, and they all have different requirements for you to prove that you are a legit film critic, not just some guy who wants to see free movies. If you can find this person, and prove to them that you are indeed press, they will send you free tickets to all of their screenings.
2. Be vigilant in looking for the promotion of these screenings. Every now and then, they will have press-only screenings. But in my experience, those are rare. Almost all of the time, they kill two birds with the same stone, by giving away free tickets to the public. This helps create a buzz, and hopefully positive word-of-mouth. These tickets are usually being given away by a radio station, or perhaps a newspaper. You just have to jump through whatever hoops they ask you to jump through, and the ticket is yours. Only problem is that it's not really a ticket. It's an invitation to stand in line for an hour, because even with your ticket, entrance is first-come, first-serve. The only people gauranteed admittance are press.