Keyboard input can be obtained in Roblox through one of two services: UserInputService and
ContextActionService. Of the two, ContextActionService is recommended as it can be easily used to bind other input types.

ContextActionService

ContextActionService binds a set of inputs to a function. To bind a function to a keyboard button with this service, call the function ContextActionService/BindAction.
For example, to print a string when the R key is pressed, a function can be bound like so:

function onKeyPress(actionName, userInputState, inputObject)
	if userInputState == Enum.UserInputState.Begin then
		print("R was pressed")
	end
end

game.ContextActionService:BindAction("keyPress", onKeyPress, false, Enum.KeyCode.R)
-- The above line could also been written as:
-- game.ContextActionService:BindAction("keyPress", onKeyPress, false, "r")

It is important to note that a function bound through BindAction will fire on all state changes of the input (began, ended, changed). If you want your function to execute on a particular input state, it is important to check for that through the inputObject which is passed into the bound function.

UserInputService

UserInputService binds functions to input via its events, such as UserInputService/InputBegan or UserInputService/InputEnded. These events fire on any input change, so it is important to check for the particular input you are interested in when handling the function.

For example, to print a string when the R key is pressed, a function can be bound like so:

function onKeyPress(inputObject, gameProcessedEvent)
	if inputObject.KeyCode == Enum.KeyCode.R then
		print("R was pressed")
	end
end

game:GetService("UserInputService").InputBegan:connect(onKeyPress)