As a beginner, you should be focusing on just riding.

It would be good to get your cadence up above 80 rpm, if possible, but you don't need to focus on that yet, and if your cadence is down around 60 or something, that may take a while ... work on it a bit at a time.

And a jump from 12 mph to 15-18 mph is quite a jump. That may not happen for a couple years ... or more.

Mainly, for now, just ride as comfortably as you can. Get a few hundred miles in. Oh, and make sure your bicycle is set up correctly for you.