What do I teach on first guitar lesson?

The six strings in standard tuning, some basic chords like G, A, D, C, and maybe the notes of the 6th string from E to G.

My 15 year old sister asked me to teach her how to play the guitar and I agreed.
The thing is I've never taught guitar (or anything actually) before, so I was wondering what the best place to start is. I'm not amazing at theory but I guess that wouldn't come up in the first lesson anyway. Should I teach her basic scales or something like that or should I get her to learn an easy song like American Idiot or something?

Maybe do some simple riffs as wel, such as the Come as you are intro or the Adam's song intro

Teach her a few warm-up exercises (that's what my guitar teacher did first). If this is her first time playing a stringed instrument, her fingers are going to ache like hell initially. Anything will do but chromatics are usually easy because, unlike scales, they're easy to remember for a beginner.

With chords, I generally found them to be the best (if very painful) way of killing the feeling in my fingertips as quickly as possible so I could progress to scales and other things.

I taught her a couple chords (A, G, C, E and D) for her to build dexterity more than anything else, I'll tell her next time to practise transitioning between them. As for warm ups, I told her to play through the D Minor melodic scale a couple times up and down. I'm not a fan of chromatics because as far as I'm aware, they aren't very useful outside of warming up, unless you're playing death metal or something.

Good idea on the transitioning thing.

Yes, they aren't very useful outside of warming up but that's how my teacher taught me so what do I know. xD You managed to teach her the D minor melodic scale on her first lesson? That's quite impressive. For the record, I don't use chromatics much anymore, I prefer scales for warming up. :3

I showed her the frets, but not the notes themselves, I doubt she remembers the whole thing. I think I might have taught her too much stuff in the first lesson so I wont be surprised if I need to repeat a lot of the stuff a few times. It's lucky though because we live in the same house she can ask me whenever as opposed to having to wait a week to see a teacher.

I'm sure the chromatic thing works well, it's just not something I used personally so I wouldn't feel confident teaching it to someone else

I did use other things in addition to chromatics as well, including something called 'crabs' and 'X's' but the principle is the same: you're just repeating a mindless pattern up and down the fingerboard that won't help you come up with anything. I can understand why you might not like it.

After a few weeks I got bored of warm up patterns lol, I ended up just playing fun riffs I like from songs I liked. That said, warming up with scales is a really good way to learn them imo, cus you're constantly playing them.

I agree. xD I've got a few scales that I can basically play when I'm asleep.

Yep, basically the pentatonic scale lol

LOL, yes. xD And also the C major scale for some reason, although that's just A minor pentatonic with extra notes.

I've been playing for a couple years and I've never actually got around to learning a major scale. I keep telling myself I will someday.

I'm not that big on theory and I like the way harmonic and melodic minors sound, so I've only ever bothered learning them

LOL, there are scales that I haven't learned as well. xD It restricts me, I think. :/ Someday I will know them all! And so will you.

Gotta catch 'em all!

I'm just too lazy these days lol, I might try and form a band when I start uni this year so I have some motivation to learn new stuff