The first thing to do is start cleaning out any leaves on the trunk, and clear the branches about 1" or a little more from the trunk out. THen once you can see all the branches, just study the tree to decide your front, then determine which branches to keep, and which to take off. This is where the art of it comes in, and you must decide for yourself what to keep, and what to take off. You should pot it in some bonsai soil, or something that drains really fast. Then, as suggested, it would be nice to take off the root that covers the others. Then maybe post more pics, and go from there. I would prune it this year, then wait until next to put into a larger pot. Are you wanting the trunk to get any larger? You could even get rid of everything but the largest trunk, or the one with the best roots, and character.