How do i get?

It's a big pain to get, I'll tell you that. To get the T-51b Power Armor, you have to complete the quest, "You've gotta shoot 'em in the head!" slightly differently than the game tells you to.

Talk to Mr. Crowley down in underworld to start the quest. He'll give you a sniper rifle and five bullets for the quest, saying that he wants you to kill four people: Ted Strayer, Allistair Tenpenny, Duskov, and Dave (From the Republic of Dave). Let's start with Tenpenny. If you haven't already, I suggest you finish the quest "Tenpenny Tower". It'll be screwed up without Tenpenny. Now, Go find tenpenny. You can either kill him, or accept his bribe and kill Mr. Crowley. If you kill him, note that Crowley will pay 100 caps if you kill him with a headshot and 25 caps if you kill him in any other way. Also note that Tenpenny doesn't have a key, Crowley just wants him dead.
Next is Ted Stayer. Go to Rivet City and find him. You can get the key that Crowley really wants without killing him if you wish, by talking to him and paying him off, or you can get it by free if you have a high enough Strength or if you have the Toughness perk. Or you can pickpocket him to get the key, or shoot him. If you shoot him, be careful, because Schrapnel might be around with a Fat Man to kill you.
Now, Duskov. Go to Duskov's place to get his key. As before, you can kill or pickpocket him, or you can pay him for the key. You can also win a Speech check to get the key, or if you're a female with the Black Widow perk. You can also get one of hiss party girls to get it off him. Whatever you do, ghead to the top right of the map for he final key afterwards.
Last is Dave. He's very tricky to get it from, as you need to pass a succession of speech checks. Or use a stealthboy and pickpocket it from him, or kill him. Grab the key from him one way or the other (And loot his office safe. On top of it is one of the rare Nuka-Cola Quantum's for the "Nuka-Cola Challenge" quest and inside is the very good rifle, Ol' Painless. Also, stop off in the museum of Dave and pick up the bobblehead on the shelf.) Now, you come to a fork in the road.
Head back to underworld now. You can give Crowley the keys for the money and kill him to get the keys back, give him the keys and pickpocket them back, give him the keys and wait a few days for him to reappear wearing the armor, then kill him and take the armor (Note that this sometimes doesn't work, as Crowley may be killed on the way.) Or my personal favorite way.
Ask around underworld and pass at least one speech check to get the scoop that Crowley isn't asking you to do this just because these people are bigots (Which they aren't, save tenpenny), so go confront Crowley about it. He'll offer you an additional 100 caps to keep your mouth shut. Gladly accept it, then save so you don't screw up this next speech check. Then ask him where Fort Constine is. When you pass, he'll mark that it's toward the northern part of your map.
Go there with the keys (I suggest taking an energy weapon like a plasma or laser rifle, preferable AC-21's Plasma Rifle from the "Replica Man" quest, as well as pulse grenades. When you show up, the place will be swarming with robots. Kill them and head to the CO offices (The smallest building in the Fort Constine area). Kill the robot inside and head down to the basement. Once you're down there, turn your attention to the safe to find some caps, the "Big Guns" bobblehead some launch codes, and some other stuff (The launch codes are just for an easter egg, you don't really need them.) Look at the cinder block at the foot of the safe to get a Stealth-Boy and a skill book. Now, open the door and head on through. With the three keys you'll be able to make your way right through this base with only interruptions from Mister Gusty's, Protectotrons, and some Sentry bots. Fight your way through the robots until you hit the lowest floor. You'll know it because there are four doors. Two lead to a weapon and ammo armory respectively, but are locked. If you have a skill of 100 in lock pick or science, go for it. If not, ignore this door. Now go to the door with a body in front of it. This is Tera, the Raider who was also hired by Tenpenny for the job. Loot her body for another key, and open the door she's laying in front of, and the next door. You'll find yourself in the room with the armor. Straight ahead is a stasis field that is disabled with the terminal on it that will release the armor (Note, the armor with the helmet weighs about 45 pounds, make sure you have room for it. Also, you need the Power Armor training to wear it). Next to it you'll find a Fat Man, two mini nukes, and a skill book. Now, head back up through the fourth and final door, but be careful, as you'll have to fight off some Sentry Bots, Protectotrons, and Mister Gusty's to get out.