How do i make a Round System Script for a game ?

First, are you sure it would be wise to make another murder game, considering there is already 4?

Anyways, the way people usually make round system is with a while loop.

while true do
    --Start the round
    wait(100) --Wait the time of the round
    --End the round
end

If you want the round to end under a specific set of circumstances, then you will have to make different events.