It seems like you're having difficulties picking up a tune by hear and reproducing it with your voice. If it's the first time you're trying to do this, it's normal and no cause for worries. Like any sensory system in our body, singing requires "calibration". If you have never sung before, your vocal apparatus may not know what it needs to do to give each specific sound or note.
To try and overcome this you need to start slowly and with easy things.
If you have access to an instrument (you should get one, if you can, or use a software instrument in your computer), play individual notes and try very carefully to reproduce each note, one at a time, with your voice, with a "ah" or "oh" sound. Start by middle range notes (e.g. around a piano's middle C for a male voice) and when you feel confident that you are reproducing each note in a satisfactory way then try to extend the range to higher and lower notes to see where your range fits.
Play little simple melodies on the instrument and reproduce them vocally. Look for (good quality) recordings of simple children songs and try to learn a few. You can do this with any simple song, but sometimes songs seem deceptively simple but have some difficulties, like large intervals, or chromatic (out of the diatonic scale) notes. Children songs tend to avoid these little pitfalls, which is good initially. Don't mind the lyrics to begin with, just use a "ah" or "la" sound initially. Allways listen carefully to your own voice and try do adjust the tuning.
When you fill confident with simple melodies, then try more difficult ones like the one you're trying to learn.