How could I make a TextBox only accept Numbers?

You can check if a message is a number by using tonumber.

Examples,

local x = 235

if tonumber(x) then
	print("Number")
else
	print("String")
end

-- OUTPUT
"Number"


local x = "385"

if tonumber(x) then
	print("Number")
else
	print("String")
end

-- OUTPUT
"Number"


local x = "Batman"

if tonumber(x) then
	print("Number")
else
	print("String")
end

-- OUTPUT
"String"

Using this you should easily be able to only accept numbers as input.