I sing and took voice lessons... it all started when I was little while dad played the guitar, as a teen I moved into solos, voice lessons, competitions and singing with pianos at weddings and such. My problem is now trying to re-train myself with the guitar. Mind you I'm only a few weeks into learning the guitar, but It's quite a feat to be able to keep the rhythm going on the guitar in the "background" while my voice is doing something else. I say it's a bit like rubbing your tummy and patting your head. With practice, we can get better at just about anything.
Record yourself singing as much as possible and listen to it a lot. Get used to hearing what your voice sounds like to others. I don't get shocked anymore... I now sound to myself as I sound to others. That's odd, but true. On the other hand, if I'd have gone by what I thought I sounded like in recording when I was first told to listen to myself instead of what others thought... I'd have quit singing long ago!
As for just singing... unrelated to guitar.... sing everything! Listen to several different radio stations and learn some other types of songs. Just sing... find time to be alone and belt it out. Then the next time someone walks in on you with your heart in the middle of a song, try to keep going... and just wait to see what they say. If they've never heard you really truly sing from your heart before, then you'll likely get an honest answer as to if you sound good or not!
ozmoid's suggestion of joining a choir is excellent. Church choirs are of course always totally free! Harmony is extremely important when singing along with anything.
As a last note... I don't think I'm an "awesome" singer or anything. I think I can sing... I think I can sing Opera type songs better than anything... sadly, I don't think I'll ever play anything like that on the guitar... so ... perhaps some campfire songs will work best for me. I've heard a lot of good music around the campfire come from people who can't sing so well... it was still great music! Don't let your voice hold you back. If it comes from the heart and you add a few beers... it'll be awesome!