How can i make a Script to choose a random player?

You can also do this in one line.

local randomPlayer = game.Players:GetPlayers()[math.random(1,#game.Players:GetPlayers())]

Broken down:

game.Players:GetPlayers() returns a table of all the players in the game

The [] brackets get something from a table, e.g.

local table = {"apple","banana"}
print(table[2])
> banana

math.random(arg1,arg2) returns a random number between arg1 and arg2

#game.Players:GetPlayers() gets the number of players in the game (# gets the length of a table)

Using the player we've got, we can put them into a team.

local randomPlayer = game.Players:GetPlayers()[math.random(1,#game.Players:GetPlayers())]
randomPlayer.TeamColor = BrickColor.new("Teams TeamColor here")