1. Some people, who "sing", must have some sort of brain damage. I say that because apparently they have no idea that if you sing an "A" your vocal chords must vibrate at 440 hertz. They can't hear tones being flat or sharp. You have to be able to hear the key. And understand that a key demands singing in the general scale of the key. So, no, not everyone can sing. It is a gift you are either born with or not.
2. If you can hear tonalities [brain OK], then you can improve your singing ability and range by training your voice to sing on key. Tune your instrument (!) and play an A. Sing an A. Sing the scale. Hole each note. Once you can sing the scale without going flat/sharp, practice expanding your range by going up a few notes CAREFULLY. It is just like pushups ... you are increasing the strength of your voice muscles as you train your voice. You don't want to tear anything!!
3. Yes you should warm up your voice. Not only will you sing better, but you will also protect your voice from damage. It can happen. Not so commonly known, is that you should make sure your sinus cavity is "clean" by desnotting yourself. Sorry for the visual. Grandma said that you wouldn't believe the horrible noises singers made before going on stage.
