Austin, Texas Area
Wall-of-text version: First thing to understand: there are no right or wrong answers to your questions. There are dozens, maybe hundreds, of ways to achieve your goals. I'm going to suggest one path, but others will probably suggest different ones. All that matters is you keep trying ways until you find one that clicks.
There are some excellent books that guide you through the creation of several full games in the Phaser engine. Practice doesn't get much easier than that!
Like Phaser, Unity has dozens of free examples and hundreds of tutorials to help you get started.
Most important, remember to start small. Arcade classics like Asteroids are a perfect place to begin. If you have favorite games from that era, try to reproduce them. Don't be discouraged if you start many games and finish just a few -- but make sure you finish some of them. Getting involved in a community -- like the one around Phaser -- may help you stay focused.
Good luck! Hope to play some of your games one day. :)