How can I get better at singing?

I sing like, all the time. In the shower, in my car, when I'm doing homework, etc. Just recently, I've thought more about my tone and voice quality, and I decided to record my voice a bit. I would do this by whipping out my phone and recording 1-2 minute voice memos of me singing various songs on my iTunes. When I listen to these later, I am fairly disgusted by the sound. My voice sounds wavy, my notes are flat, and I sound very nasally. When I'm singing straight up, I hear clear sounds and right-on pitches. I also feel as if my voice is resonating deeply within my chest and the music that comes out feels like the natural tendencies of my baritone voice. I'm sure a lot of people get this, but how many of you know how to fix it? Should I sing louder into my phone? Do I need to bring out the old piano and match each pitch individually as they come? Is there some way that I can smooth out my tone? Thanks