Hire astronauts, train them, research and improve parts. Or use the Mission Planner to create your own missions.
NOTE : This does not work on Windows XP since the "mklink" command we will use does not natively exist on this OS. Also, the Cloud can be used between different platforms since the save files are treated the same way regardless of what OS you are using. These steps will have to be repeated on each computer on which you play your KSP saved games. I prefer Dropbox since it works on all platforms and is free. This tutorial is quite short and very simple it only requires you to have Dropbox (or any similar program) and to enter a command in your computer's Command Prompt/Terminal.
A few people have found ways to make a home-made Cloud for KSP, but none of them have made a Steam Guide for it, so I decided to do it myself. Kerbal Space Program has been released after 4 years of intense development, and the developers have said that they want to minimize the use of Steam features, thus it is almost certain that they will never implement any Cloud system for the game, which is sad.