I don't teach guitar in my classes in grade 7/8, but I get a lot of students who show interest in learning to play. Here are some resources to help you teach yourself.
NOTE: Nothing is better for learning to play the guitar than studying with a guitar teacher. If you are serious about learning the instrument and can afford it, please consider this option first. Click here to search for local private music teachers.
Guitar
Justin Guitar - A highly acclaimed (and totally free) program of guitar lessons. The website is full of other useful stuff as well. Definitely worth poking around. I've linked to the page with beginner lessons.
Comprehensive guitar chord chart - A multi-page PDF containing almost every chord you could possibly play on the guitar.
One-page guitar chord chart - A simpler reference that covers most guitar chords you will come across
Ukulele
Andy Guitar's 10-day Ukulele Course - A YouTube playlist of 10 ukulele lessons to help you get started. Great for beginners.
Bernadette Teaches Music - A YouTube channel with tons of tutorials from the very basics of ukulele, to song tutorials. Tons of videos, and she's a music teacher, so she explains everything very well and breaks it down with great pacing.
Ukulele chord chart - A great reference of many ukulele chords
Ukulele fretboard chart - A handy reference for the notes along the fretboard. Great if you're trying to play melodies.
Doctor Uke - A large list of songs for beginner ukulele with chord diagrams and sound clips to play along.
Chords for Songs
Ultimate Guitar - A very popular website for users to submit chords for songs. You'll be able to find almost any song and its chords on here, even some tabs for important riffs.
UkuTabs - Another great website specifically for ukulele with a large library of songs and chords.