How can I properly use/code the 'InputBegan' event of 'UserInputService'?

InputObject

So, I tested your code and saw that 'k2' was always being printed as an InputObject.

If you look at the Wiki that contains the properties of InputObject, you'll see that it actually has a KeyCode property.

Basically, the KeyCode property will return the 'Enum' of what was pressed.

Now of course, if you tried printing the KeyCode, you'd get output such as this:

Enum.KeyCode.A

Enum.KeyCode.B

Enum.KeyCode.G

However, luckily for us, Enums themselves ALSO have properties. To be exact, they have two properties: Value & Name

http://wiki.roblox.com/index.php?title=Enum

Value and Name

Value will return the byte value of that Enum, which can be found here:

http://wiki.roblox.com/index.php?title=API:Enum/KeyCode

Name will return the 'key' which was hit: LeftShift, A, B , C, Return, ETC

The error is that an InputObject is NOT at all a String, so rather, you need to obtain the StringValue of that object. To do this, we can change that line simply to use the Value property of the Enum given to us by KeyCode.

[This is a slight cutdown on your code which I used for testing]

repeat wait() until game.Players.LocalPlayer
local plr = game.Players.LocalPlayer;
repeat wait() until plr.Character and plr.Character:FindFirstChild("Head")
local m = plr:GetMouse();
local head = plr.Character['Head'];
repeat wait() until game:GetService("Chat")
local Chat = game:GetService("Chat");
local Started = false; 

m.KeyDown:connect(function(k)
    if k:lower() == 'g' then
        if not Started then Started = true
           local EVENT = game:GetService("UserInputService").InputBegan:connect(function(k2)
            print(k2.KeyCode.Name) -- <
            end)
        end
    end
end)

The following code gave me output such as:

A

B

G

D

A

E

H

Using This

Now, let's edit your code with this!

repeat wait() until game.Players.LocalPlayer
local plr = game.Players.LocalPlayer;
repeat wait() until plr.Character and plr.Character:FindFirstChild("Head")
local m = plr:GetMouse(); local head = plr.Character['Head'];
repeat wait() until game:GetService("Chat") and script.Parent:FindFirstChild("Txt")
local Chat = game:GetService("Chat"); local Started = false; local Txt = script.Parent['Txt'];

m.KeyDown:connect(function(k)
    if k:lower() == '/' then
        if not Started then Started = true
            EVENT = game:GetService("UserInputService").InputBegan:connect(function(k2) print(k2)
                if not Started then return end
                if k2.KeyCode == Enum.KeyCode.I then
                    k2 = "i" or "I"
                elseif k2.KeyCode == Enum.KeyCode.O then
                    k2 = "o" or "O"
                end
                Txt.Text = Txt.Text .. k2.KeyCode.Name
                if k2.KeyCode.Value == 13 then
                    print(Txt.Text)
                    Chat:Chat(head,Txt.Text)
                    Txt.Text = "Type Text Here, or hit the '/' key to type"
                    Started = false
                    EVENT:disconnect()
                end
            end)
        end
    end
end)

Summary

1.) You can't concatenate InputObjects with Strings

2.) InputObjects have a KeyCode property that returns the 'Enum' of the KeyCode

3.) 'Enums' also have two properties, Value and Name.

4.) The Value property returns the byte value of the Enum

5.) The Name Property returns the actual 'thing' which was hit.

6.) You can't call 'string.byte' on the InputObject, but rather this must be done on the KeyCode's Value!

7.) You can't compare InputObjects with Enums! You must compare the KeyCode of it!

Issues to Avoid

Of course, you can use 'string.char' on the byte Value returned by the Value property, however this can lead to some issues.

print(string.char(304))
--304 is Left Shift
--Errors

That is why it is a safer option to try and use the 'Name' property when dealing with your situation. Of course, your code can be adapted to using String.char, but this is just a general warning.

Useful Links

http://wiki.roblox.com/index.php?title=API:Class/InputObject

http://wiki.roblox.com/index.php?title=API:Enum/KeyCode

http://wiki.roblox.com/index.php?title=Enum

I understand my explanation may not be the best, but I'm just trying to show how to get the actual 'key' which was hit

You should be using the events provided by UserInputService directly:

local uis = Game:GetService("UserInputService")

uis.InputBegan:connect(function(inpt)
    if inpt.UserInputType == Enum.UserInputType.Keyboard then
        --KEYDOWN
        print(inpt.KeyCode.Name)

        if inpt.KeyCode == Enum.KeyCode.R then
            game.Players.LocalPlayer.Character.Humanoid.Health = 0 
        end
    end
end)

uis.InputEnded:connect(function(inpt)
    if inpt.UserInputType == Enum.UserInputType.Keyboard then
        --KEYUP
        print(inpt.KeyCode.Name)
    end
end)