In KSP, you must build space-worthy craft, capable of flying your crew out into space, without killing them. At your disposal is a collection of parts, which must be assembled to create a functional ship. Each part has its own function and will affect the way a ship flies (or doesn't). So strap yourself in, and get ready to try some Rocket Science! The game has different game modes, you can play the Career Mode if you want to expand and manage your own Space Center, taking on missions and researching new technologies. Or you can play Sandbox mode if you're only interested in flying and discovering the Kerbal universe without restrictions. There is even a mid point between these two, Science mode.Go to: http://store.steampowered.com/ and download STEAM client
Click “Install Steam” (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don’t have).
Please follow these instructions to activate a new retail purchase on Steam:
Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam…
Follow the onscreen instructions to complete the process.
After successful code verification go to the “MY GAMES” tab and start downloading.