I would like to make a script that a frame appears in the middle above and a sentence appears?

What you're gonna need to do is make a new ScreenGui located inside of StarterGui. If you already have one, that's great. we can move on. Next, insert a Frame inside of the StarterGui. Edit that frame to whatever you want. Next, inside of the Frame, insert a TextLabel. Customize it to however you want. Now let's get to the scripting part.

This either depends on if you want it to loop, becoming visible and invisible, Use a click button, Wait a certain amount of time for it to appear.

So we'll go with this stuff individually.

Loop invisible and visible:

local frame = script.Parent -- Change to the property of your frame
while true do
wait(--Insert your number, uses seconds.)
frame.Visible = true
wait(--Insert your number again, still uses seconds.)
frame.Visible = false
end

Click script:, requires a TextButton. If that is the case, insert a LocalScript in the TextButton.

local frame = script.Parent.Parent -- Change to the property of your frame
script.Parent.MouseButton1Click:Connect(function()
frame.Visible = true
end)

Wait for a certain amount of time script:

local frame = script.Parent -- Change to the property of your frame
wait(-- Insert your number, uses seconds.)
frame.Visible = true

I hope any of these helped your case!