I've been singing my whole life. I took lessons for a couple years back in HS, but I was taught by trained singers my entire life, and I found that taking lessons was detrimental. My voice would get hoarse, and my range would decrease. Within a year of stopping voice lessons, my range expanded and I could once again sing for hours on end without even getting tired.
If you are just starting singing, you may find lessons very helpful because you will learn some of the non-intuitive techniques that good singers know. Or if you have some bad habits, they will help you diagnose and correct them.
My voice teachers also taught me a good deal about the anatomy of singing, which is good to know when you're trying to figure out why you're running out of voice, or how to treat a problem with your voice.
An instructor can also help you learn about auditions for area musicals, choirs, recitals, commercials, beauty pageants, etc. That is one thing that I miss--my voice coach kept a steady stream of audition notices coming my way, and there was no shortage of opportunities to get out and at least try to get some rolls.
Finally, and this is what I miss the most, my voice coach was also an audition coach. He outfitted me with piano recordings of the songs I would be auditioning with. I knew them, and I knew them extremely well. Instead of having to rely on a rehearsal pianist that I had never worked with before, who didn't really understand the cuts I had made in the music (and who often just improvise because they know the song already, even if it's not what you're used to hearing), I could pop in the tape and it was just like standing in the rehearsal room with my instructor peering at me over the top of the piano (the dude was like 6'5"!).
I felt the pain of that the most my first year in college. For some bizarre reason, instead of having you provide your own audition song like in the real world, they handed out audition songs. I can't play the piano well, adn I didn't have a recording of the musical, nor a car with which to get off-campus and GET a recording, so I really had no way to learn the song. I spent time banging out the melody on the piano, and I knew it well, but when I went into the audition I had never heard the full piano accompaniment and when the rehearsal pianist kicked it in, it was like a completely different song--I had no feel for how the melody fit in with the rest of the song.
I didn't even get a chorus part!
My suspicion is that I caught the tail-end of a conspiracy to keep non-music majors out of the music opportunities. The music majors would be taught the audition pieces during their voice lessons (which they were) and non-music majors would have to fend for themselves.
This mentality also screwed the department out of good singers, and screwed me out of a free trip to England when the choir went my first year because they don't publicize audition information.
The following year when I auditioned for the select choir, the director said "Where were you last year when we needed people for our England trip?"
I said "I couldn't get anyone to tell me how to audition."
He sighed and said: "Yeah....we do our best to keep audition information inside the walls of The Music Building."
My sophomore year was the same deal, but things were changing. We had to sing assigned songs, but it was a musical I had done before. I knew I would get a chorus part if nothing else, because I at least could sing the song having already sung it before.
But we had hired a new director who is something of a theatrical purist (used to direct the Royal Shakespeare Co. when Patrick Stewart was in the troupe) and didn't give a sh!t what your major was, as long as you were good. He gave me a major role!
The year after that, we were bringing our own audition pieces, and I had no problem getting parts for the next 2 years.
That's my story for you.
Keep on singing