A song's melodic range, as measured by its lowest and highest melody notes, must line up with the singer's range. If it doesn't, you'll transpose the song — move it up or down into a different key — so that it fits the singer's ability.
But it's not always enough to look at the lowest and highest notes of a song. It's also important to the core range of a song, a smaller set of notes that occur most frequently in the song and in the most important parts of the song. Musicians call this range the song's tessitura.
A singer has a tessitura as well. That's the set of notes that the singer can handle most strongly without getting vocally tired. The singer's tessitura is a smaller range of notes than the singer's maximum range. Make sure your song's tessitura matches up with the singer's tessitura.
Some singers, especially those with classical vocal training, know their range and can tell you precisely what their strongest range is. Some other singers just don't know and might even make incorrect guesses if you ask them. The way to be sure of a singer's best range is to listen to songs they've sung before and check what notes they're actually singing.
If you sing your own songs, you probably naturally place them in the best range for your voice. But if you have a low- or middle-range voice, you might have gotten in the habit of singing along with your favorite pop stars in ranges that are uncomfortably high for you, and not actually your best singing range. Don't compete with other singers who aren't even on the stage; transpose your songs down a step or two if that's better for your voice.
Some songwriters bristle at changing the melody to accommodate a singer, and some bands balk at re-learning a song in a different key, but it's a good practice to make those changes as a matter of course. Remember: If you want people to like your song, a good vocal performance has a bigger impact than any other single factor. Or, to put it more bluntly, from the audience's point of view: If it's not sung well, it isn't a good song.