Lucy,
Thank you for joining this forum. It is a good place to be for guitarists of all skill levels. We were all at the same place you are at now. Here is a link to some of the knowledge I have collected from the net for a beginning guitarist:
http://www.jcdisciples.org/guitar/beginning.html
Since you can already play other instruments you are probably starting off at a better place than I did. I 'played' the trumpet in elementry school :roll: and I was terrible. I couldn't hear myself play.
Picked up the guitar about 10 years ago and mucked around a lot. But in the last 2 years I have really improved. A good teacher (one-on-one) and always take opportunities to play in front of others. So I have played with a 'slapped together' band for a company picnic (about 500 people there), church variety shows, during worship at church, at a friend's monthly house jam, and in front of friends.
Good luck. Don't give up.