For 9 to 12 year old's a 3/4 size acoustic nylon or steel string guitar would suit.

For 13 and above a full size guitar should be suitable.

Electric guitars are not ideal for junior beginners but
could be considered for adults or after a few month's lessons.

You will also need some plectrums for learning to strum
and a small electronic/digital guitar tuner.

I currently sell guitars and suitable equipment for all stages.

If you have any further questions please ask.