Can you learn to sing just by singing?
I play the piano often, and I want to learn to sing so I can play songs with vocals. If I follow the correct singing techniques (deep breaths, straight posture, relaxed jaw, etc..), and sing along to playing songs on the piano, can I learn to sing well at one point? I've been told I sing off-tone, but my voice itself has potential. How can I go about this?
Are there any online resources that can help me ?
What does "Singing well" mean to you? If you're attempting to go on Broadway, then definitely get lessons. If you just want to control pleasing tones with your voice, with proper discretion you may be able to get away with trial and error (although getting some form of guidance will make your life easier).
Essentially, depending on what kind of singing you're planning on doing, you can easily get away with producing pleasant tones with your voice to good music.
So yeah, it depends on what you want to do with your voice.
Make sure you do ear training so you can develop subtleties in pitch difference. No one likes a flat/sharp singer.
Here are some resources i found that can be helpfull to improve your singing abilities