There is a feature called "LockFirstPerson". You could also use UserInputService to see if they pressed a key

UserInputService

UserInputService acts like KeyDown but it isn't actually deprecated and it takes ANY type of input(Clicking, Chatting, Etc).

local uis = game:GetService("UserInputService")

uis.InputBegan:connect(function(input, chat)
    if input.KeyCode == Enum.KeyCode.T and not chat then --If they press T and is not chatting/typing into a textbox then...
        print("Pressed T!")
    end
end)

CameraMode

A players has something called CameraMode. There is Classic(Third Person) and LockFirstPerson(1st person obviously.)

game:GetService("Players").LocalPlayer.CameraMode = Enum.CameraMode.LockFirstPerson
wait(3)
game:GetService("Players").LocalPlayer.CameraMode = Enum.CameraMode.Classic

Final Product

--LocalScript
local uis = game:GetService("UserInputService")
local plyr = game:GetService("Players").LocalPlayer

uis.InputBegan:connect(function(input, chat)
    if input.KeyCode == Enum.KeyCode.T and not chat then
        if plyr.CameraMode == Enum.CameraMode.Classic then
            plyr.CameraMode = Enum.CameraMode.LockFirstPerson
        else
            plyr.CameraMode = Enum.CameraMode.Classic
        end
    end
end)

Hope it helps!