Here are some important links for the Kerbal Space Program game by Squad:

Official Site: http://www.kerbalspaceprogram.com/
Official Forums: http://forum.kerbalspaceprogram.com/
Official Wiki: http://wiki.kerbalspaceprogram.com/
Offical Youtube; https://www.youtube.com/user/KerbalSPOfficial

Mod Sites:
KSP Mod Forums: http://forum.kerbalspaceprogram.com/forums/4-Add-ons
CurseForge: http://kerbal.curseforge.com/
KerbalStuff: https://kerbalstuff.com/