If you have studied any discipline — yoga, baseball, violin, acting — you know that you have to spend time working on that discipline before you become very good at it and even after you are good it takes years to be a master. A master is someone who can deliberately do very difficult things with little struggle and still be effective at a high level.
Those who think that learning to sing is about finding the “right” sound in one moment and then staying there, misunderstand the nature of vocal training and its purpose. Those who think that “vocalizing” is just a way of warming up and that learning songs teaches you whatever you need to know are equally misguided. People who assume that you can sing any song in any manner and that this is just fine because its part of “individualizing” the song are also incorrect.
Repertoire is useful for polishing something to a high degree after it is ready for polish. After Michelangelo finished the “Pietá” he polished the marble to a high sheen. He did not do that while he was working on the basic form of the sculpture. If you do the “finishing touches” while you are in the middle of a task, you will have to do them again when you are actually done. You might even ruin things so you can’t finish at all.
So it is with repertoire – songs or roles, classical or CCM — you sing to the level of your technique as evidenced by your vocal function exercises. Nothing else is possible. The choice, therefore, is to know what level your vocalizing is at and to make sure that you do not try to sing a song that is over your head. Sadly, singing teachers often assign repertoire that is extremely difficult to beginners, thinking (incorrectly) that this will allow them to “grow into” the material and teach them things along the way. No. If you take someone who can barely make it through a one mile run and put them in a 10 mile run, they will collapse in the middle or perhaps be injured while pushing past their physical limits. If, however, you let that person gradually run distances that are slightly longer than their easy endurance encompasses, they can build up to doing a 10 mile race and maybe winning it. In fact, if they are to coast through the ten-mile boundary, they should train to run at least 15 miles in hard conditions over a period of time. THAT would make the 10 mile race much easier.
If you can barely make a sound that is secure, free and undistorted, at a moderately loud volume, in your highest pitch range, then singing a song with those requirements, particularly if they happen over and over in the song, is doomed at the outset. You can only struggle, push, and ignore the extra effort you are expending and that, more than anything else, will actually set you back rather than help to develop your vocal skills.
When evaluating a song looking at the pitch range, comparing it to the normal pitch defaults of vocalists (SATB and all derivatives thereof), at the tessitura and the lyrics (how emotionally potent are they?), the tempo (how fast or slow) and the style (ballad or driving pop/rock song) can give you a lot of information, even if you have never heard the song or seen it as printed music before. When you choose or suggest repertoire for a student do not pick songs that are more than a small amount beyond the student’s present skill level. In fact, the song should be easier than the vocal capacity of the singer as evidenced in vocal function exercises. Even if the song is very easy, making the singer dig into the meaning, the communication and the nuance of the style should be challenging enough. Even “Happy Birthday” can be interesting. Remember Marilyn Monroe at JFK’s birthday long ago?
If you want to push the person, make the exercises more challenging, but not so hard as to wear the person’s voice out during practice. If you can’t make informed repertoire choices, go learn how to do that. Don’t guess.