What are some very basic, beginner tips for photographing an event?

My sister has asked me to photograph her graduation since no one else in my family has a camera. I have a relatively nice one, it's a Nikon D3100 with just the lens that came with it.

I have never taken pics of anything other than my friends at parties so really any helpful tips would be nice. Like what flash should I use for indoor and outdoor, what angles, whether to manually focus or just use the auto…

Thanks, any help is appreciated.

Added (1). Oh even any tips on retouching would help. Like what tools to use on photoshop.

Sounds like more of a casual job since it's for your sister, but definitely make it nice! So I would say going full auto should be fine since cameras nowadays (especially DSLRs) can automatically adapt to quite a lot of different situations. But if you really want to go "pro" manual, try adjusting the ISO and White Balance. In terms of focus, I would say auto is fine since you won't have too much time to manually focus, wouldn't wanna miss those quick moments! On photoshop, I would say most of the adjustments would be on the color level, exposures, saturation/hue and etc. Which is accessible on the: image ---> adjustments I believe.