Would it be a big mistake to stick to my cameras auto mode?
Yes.
If you're not comfortable with full manual, try going at it with Aperture... but even then, if the venue is super dark you may have some issues with the shutter speed slowing down too much, so keep an eye on that.
As Kitty something or other (there are too many kitties in here now...

) said... stay close to the stage. Don't get shots where the mic is completely covering the singer's face. Singers are best shot at from an angle, rather than straight on because of the mic...
Don't be afraid of high ISOs... You're probably going to need to be *at least* at ISO1600... and that's if the lighting is decent.

If it's during the day time (just realized you said the stage will be outside), then you probably won't need to go as high.
It's really kind of hard to give you advice on what to do without having any specific questions to answer. I'm just trying to think of what I had issues with before...
Something that came up in another thread was matrix metering vs. spot metering...
If the stage is evenly lit, you can go for matrix, but if it's really spotty or dark, spot metering is going to be your friend.
S'all I got for now.