It's a money tree, Pachira aquatica. The specific name is a good clue as to what conditions it should be kept in. Naturally, it comes from tropical wet lands and swamps. It will need a high organic bonsai soil, one that includes humus as a good portion of its consistency. After that, calcined clay, lava, pumice, horticultural sand whatever are all fine to use and mix in. Now, because it's a wetland species you can assume care to be similar to other wetland species, that being the soil should never be allowed to dry. But different wetland species live at different point in the water table. Some can tolerate having their "feet wet" constantly. What that means is that, in the soil, they're also roots deep in standing water. Pachira is NOT one of those species, and so should never be left in standing water, but should be watered again before the soil dries.
The really good news here is that, treated well, this species is very hardy and difficult to kill. The bad news, as Norm stated, is that this is decidedly not bonsai in the artistic sense.
As far as where to go with it, well, just keep the style it's in, nothing else will work at this point. As for proceeding in bonsai, that depends on where you want to keep the trees.