I started playing about 3 weeks ago, so I have a recent learning curve. I just finished a 9 person Mun landing (damn tourists) and am about to land a science base on Minmus. 1. Started in career mode. Limited availability to parts and missions ensured I was not overwhelmed. 2. Suborbital flight taught me about gravity turns, A&P and Prograde/Retrograde burns. Orbital flight taught me to circularize and de-orbit. Re-entry was pretty easy at this point. 3. Changing orbits is where a large chunk of learning occurred. Getting a kerbal stuck in a polar orbit was tough love on how much dv it takes to change plane (eventually learned to launch into a polar orbit). Normal and anti-Normal, fine turning orbits, and changing the plane of my orbit forced me to learn to use the Nav Ball and fiddle with Maneuver Nodes. 4. Rendezvous missions was the last major mechanic I learned and I am proficient in them. 5. Mun intercept was pretty easy (see the Mun, burn Prograde! Now I use a Maneuver node but the simple version worked fine at first). 6. Landing on Mun/Minmus was pretty straightforward (I use the mod that gives me data, but don't use MechJeb) 7. The next thing I will learn will be gravity assists and planetary windows. The resources I have used to learn so far: Kerbal forums/tutorials, YouTube (mostly Manley video's, but I will often run any interesting ksp video on my second monitor while playing), ksp wiki, Google. The hardest thing to learn has been the interface issues. e.g. How to gather science. The rules around material bays/storing science are pretty quirky when you first run into them. Even now I often reset the bay just to realize I deleted science I then need to collect again. Also, I have gradually increased using all the tools in the VAB and probably still have many more to learn/get comfortable with.