How do I define what math.random was chosen?

Each time you call math.random, it will give you a new number[^1]. If you want to keep the same number, you need to save it to a variable:

local choice = math.random(1, 2)
print(choice)
if choice == 1 then
	local message = Instance.new("Message, workspace)
	message.Text = "1 was chosen"
end

• Remember that == means "is" and ~= means "is not"
• Remember to tab your code correctly
• Remember to use local to define new variables
• math.random(n) is short hand for math.random(1, n)

Also, if you're just going to say "x was chosen" for each number, there's no reason to use an if at all:

local choice = math.random(2)
local message = Instance.new("Message", workspace)
message.Text = choice .. " was chosen"

[^1]: Often when you have computations, you can pull out a value and replace that value for the computation. E.g., math.sqrt(4) and 2 are always going to be interchangeable because math.sqrt always returns the same thing. This is called referential transparency. Because of this property, we say that math.sqrt a pure function. math.random is NOT a pure function, because it changes each time you call it[^2].

[^2]: Pure functions are usually easier to reason about. For things like randomness, it's often much more convenient to produce different values.