To start, I'll let you know the basics. In MMD, you can load and use either Vocaloid Accessory Files (.vac) or DirectX Files (.x) in MikuMiku Dance. DirectX files are the more popular solution because most 3D software has the ability to export to it by default, sometimes with an added script.
If you find an accessory either gives you an error pop-up, and the file doesn't show up, there's a problem with it. If it does show up but doesn't have textures, it can be one of two things. The textures are in the wrong folder, or your computer doesn't support the newest version of MMD. If you're running XP or later, this shouldn't be an issue.
Don't worry about your level of skill right now. With time, patience, and most importantly commitment, you'll steadily learn and grow as a modeler. Try looking at demos and process work. Looking at meshes can allow you to discover how others make things fit together, and ultimately give you tips and tricks to try yourself. This will help you develop your own style.
Installation varies, but your program will usually have a 'plugins' or 'scripts' folder somewhere in it's respective folder (sometimes they're hidden, or you can try creating your own). Scripts just get dumped inside there and load the next time the program launches. Add-Ons or Plugins sometimes need to be installed separately, but they'll typically unload in the same directory as your program.
Generally, you might see anywhere from 1000-7000 vertices. You shouldn't have anything crazy like 'millions'. It doesn't even have to be high poly to look high poly in your 3D program. MMD Will automatically smooth out all of the surfaces so you really don't need to do much besides model, color, and texture it. Try to avoid adding fancy effects if you're exporting as a .X file because this could cause problems in PMD or in MMD.
For information about modeling, please see the 'Generic 3D Modeling' tutorial.
Choose what you want to MakeDepending on the complexity of your project, you may want to plan it out beforehand or use some kind of a reference. If you're more comfortable with your skills, have a good idea of what the object looks like (if you draw well, you should have an advantage), or only intend to make a simple object, you could get away with winging it.
You could go without a reference, as long as you have a clear mental picture of what you want to do. However, being able to use this method is a skill that takes practice, and above all patience.