What variety is it?
There are as many different strategies as growers, but I would let this tree reach at least 1/2 and probably 2/3 of its "finished" thickness (caliper) before you cut it. If it is far from that, you might want to plant it in a growing box or the ground.
Depending on how you intend on styling it, you may want to change the trunk angle. Don't worry about the existing branches unless you have a grafted maple - these originate much too high to be concerned with.
So, you need to decide at some point before cutting it:
What style will this tree be?
How large will this tree be?
You may want to consider subscribing to
http://www.bonsaitodayonline.com as there is an interesting article in there about drastic pruning that describes some pruning methodology and reasoning.
If you are a subscriber, the link is here:
Trunk Chop 101 for Deciduous Trees