Find the key of all the songs you sing. There are any sources on the internet. Type out the lyrics with ample spacing. If you prefer, use music staff paper. Listen to the first few notes that are sung. Vocalize them along with the artist singing to make sure you have a good start. Go slow and figure out the next note. Sound it out. Did it go up a whole step or two whole steps? Sound the notes on a keyboard. Mark down the notes. Write their letters on the lyric sheet under the words being sung. Then repeat the process until you’ve done the whole song.
You need to practice Singing the melody 100 times. Classical pianist Vladimir Horowitz would stop only after he played a song 120 times without any mistakes. Then tune your voice to the melody while playing the notes on your keyboard.