I would say find a cadence you like, and get proficient at changing gears often to stay within that cadence. What feels comfortable for you now probably is the most efficient for you now. After a while experiment with changing cadence a little. You may find that you can spin a little faster and that it has some positive effects, or maybe you find it's not worth the hassle. Unless you're racing there's no need to stress about it. I don't have a cadence meter, but I know that my cadence is a bit faster now that when I started riding, and it gives me a little more leeway to adapt to quick terrain changes.

There's no "right" speed, it all depends on terrain, wind, riding position, group/solo, weight of your bike etc. 24 km/hr sounds great to me for a first ride.