How do you get better at singing?

Practice.

What I suggest is sing along out loud with songs you like. You can find where your natural pitch is and through experimentation self-teach yourself how to harmonize with other vocal pitches.

Humming doesn't count! And no singing softly, either.

Learn to breathe from the abdomen for maximum lung capacity. Try doing do-re-mi scales up and down on a keyboard (you can find virtual instruments online.)