To get really good, you'll need to put in lots of hours developing. There is no shortcut that I'm aware of. You might as well make those hours enjoyable though.
Find where your interests with development overlap. Seeing as you already have a background in Math and Physics, put it to use. Go look for some open source projects that you're genuinely interested in and try them out. Like other people have said, contribute and engage with the community.
If you're just fixing small bugs in these projects - it's still valuable enough in that you had to have some understanding of software some other people wrote (in a potentially large codebase).
If you do eventually decide to be more ambitious with creating a project of your own - aim to do something very useful / interesting (that doesn't necessarly mean a large application). Make it open source, have people review it and don't shy to promote it. That kind of thing is great to have on a resume.
Find where your interests with development overlap. Seeing as you already have a background in Math and Physics, put it to use. Go look for some open source projects that you're genuinely interested in and try them out. Like other people have said, contribute and engage with the community.
If you're just fixing small bugs in these projects - it's still valuable enough in that you had to have some understanding of software some other people wrote (in a potentially large codebase).
If you do eventually decide to be more ambitious with creating a project of your own - aim to do something very useful / interesting (that doesn't necessarly mean a large application). Make it open source, have people review it and don't shy to promote it. That kind of thing is great to have on a resume.