you buy the game , install it. i recommend you get the 10th anaversay edition then hope you can either find an active server or some1 you can link to by entering there ip address.
http://europebattle.net/ra2/ some private red alert 2 servers. you will be lucky if your thinking the original red allert. the game is so old and the company that made it went under a long time ago. (westwood studios)