What is the code for a and d keys?

Why not just use UserInputService?

They are very simple.

Do this in a local script.

-- If working with the player
local player = game.Players.LocalPlayer
-- If you want to receive mouse input
local mouse = player:GetMouse()
-- If you are working with the player's character
local char = player.Character
-- Crucial line of code when working with the character
repeat wait() until player.Character

-- Store User Input Service in a variable
local UIS = game:GetService("UserInputService")

-- Handle user input
UIS.InputBegan:connect(function(input)
    if input.KeyCode == Enum.KeyCode.A then -- if user presses "A"
        -- Do things when "A" is pressed
    elseif input.KeyCode == Enum.KeyCode.D -- if user presses "D"
        -- Do things while "D" is pressed
    end
end)

Keep in mind that if you would like to detect when user input has ended you will need to use the InputEnded event. If you are using loops then you will need bool values to detect when input has ended and stop the loop.

Note: I wrote this code directly in the browser. Don't be surprised if there are errors.