I'm glad you enjoyed it.
It took me about six months of dabbling in MMD to figure out what's what. There are great tutorials out there that are very easy to find and play along with, but I immersed myself from the onset.
I started watching youtube videos and picking the songs/animations/cameras/characters that I really liked. (My reasoning was: If I was going to spend an hour or so modifying things around, I might as well not kill my ears to something I dislike in the process.) Once I found a good video, there were often links in the comments to where those base files (song/char/cam/motion) could be downloaded.
Then I just mixed and matched, putting Character X into Music Video Y. If I wanted to include something like a potted plant or a ruler for scale, I'd figure out how to manipulate the x/y/z coordinates, and that introduced me to manipulating character movements over the course of the number of frames I'd deigned.
Addons were big. I learned how to use the ScaleG accessory, which meant that shrinking and growth modifications were in my hands. I learned about PMX Editor, which allows the creation and modification of the 3d models; it also allowed for isolated manipulation of specific pieces of those models (so that meant breast expansion and uneven size difference were available).
Really though, just to start and not get overwhelmed, just go find an MMD music video you like, get the relevant music file and a 3d model you think looks nice, and a background stage that you find interesting. Then download those, throw them together, and enjoy how it plays. Later, change the model. See how it works out.
But always have fun.