#2 If you don't have a blizzard account you can create a free account using the link I provided above. It will say [Support] & [My Account], proceed to click [My Account] for a drop menu, then find & click [Create Free Account]
#3 Open the Desktop Blizzard Launcher (make sure you sign in)
#4 At the top of the Blizzard Launcher, it shows [Games] - [Shop] - [News] Proceed to click [Shop], next scroll down until you see [Blizzard - Classic Games] on the left side menu & click on it.
#5 Warcraft III will be show here, Reign of Chaos is the first game, The Frozen Throne is the expansion (you need both)
#6 Once you purchase both Warcraft III games the Blizzard Launcher should automatically install & update them for you. If not you will need to manually find where you downloaded them to & install them manually. (Default location is in the Downloads folder)
If you need anymore help with this I will have to refer you to Blizzard with links provided below.
Classic Blizzard Games Discussion Forums Link: Here Classic Blizzard GamesBlizzard Support Forums Link: Here
If you need help on hosting Warcraft III games you could check my support thread. Here