You have to go to the 'Download It Here' section on the right side of the website minecraft.net.

Click Minecraft.exe and you will get the launcher for minecraft. After you download the launcher,

you will get this website on your screen (Might be Minecraft.net) where you have to make a

username. If you played the demo version, you don't need to make your password, since you

made one already. If you didn't, you need to make one when you have the website on your screen.

When you're done you could start playing minecraft. (Move the application to your desktop.)