Gain control over your voice by learning to sing major and minor scales. Play each note on the piano and match it with the voice. Singing scales will strengthen your voice and make it easier to match pitch.
Play a single tone on the piano and attempt to match the pitch. Use the damper pedal on the far right to extend the sound of the pitch. Try to make your voice go in and out of tune by singing slightly below the pitch and then above the pitch. Listen for beating within the two tones. Once you have exaggerated the intonation to a point where you can hear the beating, it will begin to get easier to hear any pitch that is out of tune.
Develop your ability to sing in tune by carefully listening to the interference beats whenever you sing in unison with another instrument. Make recordings of your songs on the piano so that you can sing along with them and concentrate on the pitches.