This can potentially be complex, but to make it simple:

Biggest ring on front when on flats or decents

Middle ring when going up light to medium grade hills or on windy days

Smallest ring when going up very steep hills

Cycle through the rear cogs based on feel or desired speed (lower gear when accelerating from a stop or speeding up after slowing down, and bigger gear when you want to reach a faster top speed)

Cadence is usually considered "best" between 80 and 95 RPMS but of course varies depending on the situation, but there isn't a rule saying that it must stay constant.