I've taught around fifty students the guitar up to grade three.
Essential! Don't worry about not being able to do this at the start, it will come with practise. Lots of young guitarists need helps in this respect, especially when starting out. Guitars require tuning every time they're played.
A cheap tuner will be very useful: Tiger sells one for about £6. Free apps are handy. The books I've recommended on this page come with tuning tracks on their CDs, which is actually a better way of tuning than by using a tuner since the guitarist learns to listen and tune up based on what they hear rather than see on a display. The same goes for the following video:-
Buying a guitar
I tend to buy both second-hand and new guitars in person, usually at Harmony Music in Dorchester and the music instrument heaven that is Absolute Music in Wallisdown.
I recommend the following books for learning to play guitar:
I recommend Trinity Rock and Pop exams and also use RGT and Rockschool.