This is perhaps a condensed version of the advice above, but it has served me very well through the years.
When I bought my first street bike at the tender age of seventeen my father, an avid cyclist himself, knew I was already fairly adept at motorcycle operation from years of enduro riding. He made certain that my first road machine was a small, forgiving bike, and spoke the following sentence, which I keep in mind when operating a car, motorcycle, or bicycle:

"Assume that everyone you see will try to kill you, and ride accordingly"

After that, he turned me loose.

So far, so good. Paranoid? You bet, but I've still got all my original parts.