There are several inworld tools for sale that let you put your avatar through a series of poses that you can then upload as an animation. They are fairly simple, but effective. I'd recommend getting a bit of experience with one of them. There are basics to learn about initial-offsets and such. Its easier to learn the fundamentals first before moving to a system like blender that has many more degrees of freedom.
Speaking of Blender, CG Cookie has a good free beginners tutorial. Watch that before deciding if a CG Cookie subscription is worth it for you.
Speaking of Avastar, the @Gaia Clary 's blog keeps one reasonably up to date about the progress of Avastar for Blender 2.8/2.81. Avastar for Blender 2.79 is an incredible tool, but if you are just getting started I recommend spending the time learning Blender 2.81, and then you'll be ready for Avastar 2.81 when it launches.