Whether you can teach anyone to sing really depends on the skills they have already. There are some people out there with a natural talent, there are some people that aren’t brilliant but if you train them hard enough then they’ll get it and then there are some people whose voices will never be tolerable.
The place to start is the range. The range identifies the highest and lowest note that they can sing comfortably, anything between these notes is their range. Once you’ve identified the range that someone can sing comfortably you’re half way there. Despite there being some great singers out there, their abilities are far from limitless and ask them to sing a song that’s not within their range and they’ll sound just as bad as the next person.
Next comes the ability to sing in tune. This must be learned as, unless you are a master of music manuscript, it’s unlikely you can sing based on notes. Instead singers use tone. This means that you have to listen to the note and then sing it. After this you should sing the subsequent notes in tune too as they are relative to both the backing track and the previous not that you have just sang.
You don’t need to be pitch perfect to be a good singer. Some musicians have an ability to recognise or sing particular notes down to a few hertz of accuracy. The note most commonly used as a reference is the note A which is defined as 440 vibrations per second. Some people can accurately sing this note without any reference (i.e. being prompted to match a note of a tuning fork for example). Perfect pitch is not necessary though; singing a tune starting at 400 hertz (in between recognised notes) would be considered fine if the next note was of a frequency that was correct, due to being relative to the previous note.
A great way to get the perfect pitch is to use any instrument to produce a single note and then use your voice to try and match it. Computers are also a great way to do this; you’d be surprised at the number of computer program that can create and play music on any instrument. There is also software where you can create virtual bands. When you’ve matched the right note it should be obvious and the more you practice the easier it will become.