We don't know where you live, so it would be hard to tell you if this is the beginning of a fall color change. But zelkovas will drop their leaves so take a look around you and see what the elms might be doing. You'll want to keep your tree outside - at least while it is actively growing - some people overwinter in the garage during the colder months. Again it depends on where you live.
You'll want to pick up a general book on bonsai (Sunset's BONSAI magazine is good) and perhaps talk to some people from a bonsai club in your area. Try looking at
BCI Species Index for some basic care guide.
Regards,
Matt