Your first electric furnace should be a Lil' Bertha style with the groove in the furnace wall and a coiled heating element. I say this because its quick & easy to build and if something goes wrong it will be easy to fix.
For a heating element, use 17 feet of 18 ga nichrome wire. This will be 8 ohms which will be very close to 15 amps draw on a 120 volt supply. Don't plug anything else into that circuit while melting! If you have an ohmmeter (or can borrow one) you can scavenge heating elements from dryers and other electric heating devices.
Performance of your furnace will be enhanced by using insulating refractory. If you do not insulate, be prepared for a 2-3 hour warm up time on your first melt of the day. Also, attaching wiring to the furnace exterior becomes problematic because the heat will cook your wiring, switches, etc.
Personally I started out with lots of controls on my electric furnace but quickly graduated to running the element wide open all the time. An electric furnace already melts slow enough. You will be thinking of ways to get your furnace to melt faster! You will also find that there is a 10-20 minute "window of opportunity" between "barely hot enough to pour" and "almost too hot to pour". Personally I just plug mine it into the wall to turn it on but if you have to have a switch an ordinary 15 amp light switch works fine.
I just built an element control for another project (vacuum former with an 8 ohm nichrome element) the way Gingery says to. Spent $60 on components and its pretty much useless. Even forming a soft, quick heating plastic like Vivak best results are obtained with the control on its highest setting. If you just gotta have a control, you could buy mine.......
-- Mark firstname.lastname@example.org