Sing a song that you know well. Record yourself as you sing.
Listen to a professional recording of the same song that you sang yourself.
Play back the recording of you singing the song and compare your rendition with the professional one. You don't have to sing extra embellishments and accents that a professional singer might add, but the basic notes of the melody should sound the same.
Ask someone with a musical background to listen to your recording and help you to analyze if your voice was in pitch. Isolate any inaccurate sections of the song and practice them until you can sing the correct pitches.
Play series of pitches on a melodic instrument, such as a piano or xylophone. Hum or sing the pitches with a syllable like "la" as you play. If you can't play an instrument, ask someone who can play to do this for you as you sing the pitches. You can use the instrument to isolate difficult parts of songs as well. Break the sections down pitch by pitch until you can sing through the section accurately.