I started performing at 15. I'm 63 now... here is the short version:
#1 - you can't really practice performing. You have to do it to get good at it. The more hours you put in in front of an audience the better you will get at it, and sadly, there is no other way. Do it as often as you can.
#2 - Performing is at least in part an acting job. Act like you're very calm and cool. Act like you're not nervous. Act like you know what you're doing. You're the one on the stage in front of the room. Be entertaining... tell a story. Look people in the eye, not over their heads, not at your fingers, not at a music stand.
#3 - Pick out some friendly faces and talk/sing to them. Look at others but come back to them. Seed the audience with a few pals. Connect, they're just folks. You can't hide up there, so don't try. Be bold.
#4 - Know your material backward and forward, including any banter, stories etc. Someday they will become 2nd nature and you won't need to think about it, but at first learn your stuff thoroughly, because when you get on stage you will forget half of everything you ever knew.
#5 - Pick material you love... songs that really means something to you. Songs that move you. What you want to communicate is the FEELING you get from it. After all, that's the whole point isn't it?
#6 - Every song tells a STORY. You need to know the words, chords etc so well you don't have to think about them, so you can concentrate on telling the story. It's like driving in a way... you can't think about where you're going if you're concentrating on the operating the vehicle controls.
I studied for a while with a stand-up comedian because I wanted to be more comfortable between tunes and a little funnier. If you can get an audience laughing you've got them. I think I have a good sense of humor but on stage it's a different animal. We like to think that comedians are making stuff up as they go, but they're not, they're acting like they just made it up. I ended up doing some short sets at a local comedy club (without a guitar) and much to my amazement it wasn't as hard as I thought it would be, but it took hours of prep work. if you can live through that you'll be in good shape.
The main thing is to do it as often as you can, and when you get up there try to relax, take your time... breathe, look around, smile! The audience is your friend. Be yourself.