Welcome WitchDoctor!
The Bonneville America was my first bike after about 8 yrs not riding. I remember I was a bit apprehensive at the time, not sure what style bike to go for, cruiser/ commuter/ sport/ tourer? so many options, make/ style and even whether I'd make the test ride or not.
In a way it was like learning to ride all over but quickly found no problem getting back in the saddle again with the America. It's up to you what bike you end up going for, after your preliminaries try a few and see what you feel most comfortable with. IMO I found the America very forgiving, easy to ride and get used to after a long period not riding.
As for training, try and get as much input as you can. Official courses, books, internet, whatever, it all helps. Some great posts on this site.
My short advice would be to take it easy, gauge your speed to the road conditions/ traffic, leave plenty room between yourself and the next vehicle, expect the unexpected - ride like you're invisible at every junction, try to plan escape routes, be 360 degree aware and try not to overtake near junctions, especially don't overtake at crossroads. This is just in case BTW, usually riding equates to much fun
