Tutorial:Sharing craft files
You've shared a picture of your new super awesome rocket, maybe even a video. How do you share the rocket itself?
All your ships are stored as ".craft" files. They're very small text files and can be shared easily.
If you have parts that are not part of the stock game (such as from MechJeb or Kethane), please either remove them, or if they're essential to the ship, mention the addon(s) and/or plugin(s) needed when you post the .craft file.
Finding your .craft file
Where is your super awesome rocket? Assuming that your super awesome rocket is named super_awesome_rocket in the game, that it is built in building (insert VAB or SPH), and that you are saving under save_game_name, you'll find your .craft file under "saves" in your KSP root directory. So the path is ROOT DIRECTORY/saves/save_game_name/ships/building/super_awesome_rocket.craft. If a save game with that name was created, all directories should exist. At least the saves directory must exist, otherwise you are in a wrong directory.
Uploading your .craft file
Now that you've found your .craft file, how do you share it with the world? There are many file sharing services!