Obviously you will need a motor, controller, and batteries.
Other things are a battery charge monitor (essentially a fuel guage).
A 12v converter to step down the series battery voltage to power the indicators and lights, many people just take it directly off one battery, but this loads one more than the others, so will wear it out earlier, especially if u ride with headlights on.
A good battery charger, some can charge straight to the main voltage, while others need to be connected to each cell individualy.
How they connect is usually fairly self explanatory, as long as you get matching components, suppliers will be able to help you out there.
Bike frame depends on your design (batteries and motor placement) and mechanical aptitude, i recomend a diamon tubular aluminium frame (common on 90's bikes), if you can weld aluminium of course. This type of frame means you have a large clear space lower down, and requires only the addition of more material, which will not compromise frame strength, this is the frame im using in my current conversion (see high performance homebuilt sportsbike). On the other hand the double cradle frame type (80's bikes), allows for less mechanical work, as there are frame supports down low, however these are generally unable to be moved so if your parts do not fit well then you're buggerd.
hope this helps