If you are a primary school student and want a guitar, then you'll need something 1/2 or 3/4 size that is easy to play. If you are pre-school then you'll probably do better with something even smaller like a little ukulele.
If you are starting late and you're all grown up, then you might already have decided you want to buy an electric guitar or steel string acoustic.
Whoever you are, and assuming you are an absolute beginner, I'll recommend a few options here.
Very small children need expert guidance with even the smallest of stringed instruments. The best advice I can give is to find a local teacher who can help musically assess your child and make a decision about the best way to start after that.
For children between the age of 7 and 11 a half -size nylon strung guitar is usually the right choice. A 3/4 size can be suitable for taller children and secondary school age students.
The guitar must be small enough not to be cumbersome and awkward to play - and have nylon strings that are soft on the fingers and good for learning finger-style or plectrum style.
I recommend any one of 3 guitars to my students. All 3 play and sound well enough:
You pretty much get what you pay for - the Yamaha and Cordoba guitars being the far superior instruments and great value. I use both of these guitar to teach on my home visits because they are more portable than my full size instruments. The Valencia is the loudest of these 3 options which can help children learn to play more dynamically in group settings.
Guitar Cases
The cases that come with the cheaper guitars are not usually much good at all. So I'd recommend buying a separate case that is a bit more protective and that has a document pocket for carrying music and books in. Most music shops that sell 1/2 and 3/4 size guitar will have cases, some padded, some not, some waterproof, some not. Children who take their guitars to school, really need more robust cases that will protect the guitar when it falls over, is kicked or trodden on - accidentally of course.
If you are spending £70 or more on a guitar, I recommend this protective carry case for a few good reasons. It is padded, waterproof and has a large enough pocket for A4 music and books and more. It also has good straps and is easy to get in and out of cars. It also comes in Red, Blue or Black colours. The bad news is that it is about £30. Any case that is truly fit for purpose will cost around £20 though.
Gewa 212121 Tear Proof and Water Protected Economy 1/2 Classical Guitar Gig Bag
Cheaper Guitars
There are lots of other cheap guitars out that may play and sound as good as the 3 I have listed here, but there are lots that certainly don't. If you can't find stock or need more choices then feel free to contact me for help.
If none of these options work for you then go into a good music shop and find something there rather than risk an online purchase from Amazon or wherever. You don't need a strap, but you will need a music stand and a foot stool which combined should cost less than £20.
Before you buy anything, think hard about why you are learning and what your goals are. If all you want is to sit in the garden with friends and strum a few songs to sing along to, then you'll want a different beginner guitar than someone who wants to be the next Jimi Hendrix.
Whatever you buy it needs to be easy to play and soft on the fingers. I always recommend nylon strung classical guitars for learning on, especially if your goal is to strum songs and sing along.
If you want to learn rock, blues, jazz or any other modern guitar techniques that generally mean you'll play mostly with a plectrum, then an electric guitar is fine to learn on too.
If you're local to me and would like me to help you find a guitar by visiting a local guitar shop together, then feel free to contact me - spending time in guitar shops is always something that makes me happy.