Okay.... a couple of things here. A good singer - depends on what you're judging. You can have a singer that is pleasing to listen to for whatever reason (can hold a tune, has distinctive voice with a quality that you like, etc.), but that does not make them a good singer. I think of Kurt Cobain, Rod Stewart, Brian Johnson. I love their voices, but I don't believe they are good singers. Why not? There seems to be little technique, lots of straining, and not much resonance in their tone. All of these things lead to problems later. (I know I sort of contradict myself there... If anyone knows how Brian Johnson can even TALK these days, I'd be interested in knowing....) Cobain didn't really have time to mess up his voice really badly, and Rod Stewarts vocal problems are well documented.
A good singer, IMO, will be able to reliably perform on any given day, from now until old age. This involves techniques that you can use to NOT strain the voice, and to have a resonant (not necessarily operatic) tone (which is closely linked to projection/perceived strength in the voice).
Why is giving advice for vocals any different than giving guitar advice? A lot of guitar advice is not "technique" advice per se - a lot of it revolves around play these scales, do this for a warm-up, etc. Technique, as I see it, is the PHYSICAL framework within which you perform. As a guitar player, technique will probably take the form of (but not exclusively limited to) good hand position (finger tips perpendicular to fretboard, fingers parallel with frets, thumb pointed at ceiling but not over the neck, etc. YOu can SEE those things, evaluate your progress, and make changes accordingly. With singing, yes, the difference is that you can't see your technique - though there will be some visual cues to suggest whether you MIGHT be doing it right. How will you know if you're lifting your throat? How will you know if you are, indeed, focussing your voice across the cheekbones? How will you know if you are supporting your breath properly? How will you know HOW to rectify any problems you might encounter with those things? Answer...you won't without an instructor who knows how to spot those things.
(refer back to swimming analogy....)
Chris