Choosing a guitar
There are 3 main types of guitars; acoustic, electric and classical. The best type of guitar for a beginner guitarist is a classical guitar (with nylon strings). Other types of guitars can be difficult to play, as the strings are hard to press down, especially for younger students.
Classical guitars come in different sizes. Here is a rough guide to which size you will need:
Ages 4 to 6, height 3'3" to 3'9" (99 cm to 114 cm): 1/4 size
Ages 5 to 8, height 3'10" to 4'5" (117 cm to 135 cm): 1/2 size.
Ages 8 to 11, height 4'6" to 4'11" (137 cm to 150 cm): 3/4 size.
Ages 11 and above, height 5' (152 cm) and above: 4/4 size. (Full size)
HOW MUCH TO SPEND
You may be able to find a very cheap guitar online, but beware; very low cost instruments can be poorly made and will be hard to keep in tune. This can make practising difficult and may put the student off learning altogether.
Here are some recommendations for new guitars (3/4 size versions)
£50 - 70: Valencia VC203-VNT
£100: Yamaha CS40
If you want to spend less, you might want to look for a second-hand Valencia or Yamaha rather than buy a less expensive new guitar. Remember to make sure the guitar is a "classical" or "nylon-stringed" (not "acoustic"), and is the right size, before you buy!