In order to purchase Microsoft Flight Simulator X: Steam Edition from Steam you will need to have a Steam account. If you do not have an account please follow this link and create one:

https://store.steampowered.com/join/

Once you have a Steam account login to the main Steam website:

http://store.steampowered.com/

Then head to the Microsoft Flight Simulator X: Steam Edition game page where you can purchase the game:

http://store.steampowered.com/app/314160/

Once purchased please follow the on screen instruction as detailed by Steam.