If you are picking up a guitar for the first time, it is important to first decide what kind of guitar you need. There are two main types of guitar: acoustic and classical. The difference between them is both construction and the strings they use.
A classical guitar has a shallower body and uses nylon strings, though the bottom three strings are wound with silver.
An acoustic guitar has a deeper body and uses steel strings, in addition it has a plate positioned on the body below the strings to protect the guitar from plectrum damage, as you can see on the pink guitar shown on the left.
Acoustic guitars come in right handed or left handed versions, as the plate has to be fixed the right way round. Most players use their dominant hand to pluck or strum the strings and the other hand to place their fingers on the neck of the guitar.
Classical guitars do not have right handed or left handed versions - as they have no plate they are fully reversible and all the player has to do is restring it the other way round.
Young children usually start on classical guitars as they are smaller and the nylon strings are easier for young fingers. So they can play acoustic style guitar on a classical model. Classical guitars come in 1/2, 3/4 and full size (4/4) which make them suitable for very young ones, whereas acoustic guitars generally only come in 3/4 or 4/4 sizes.
A classical guitar is shown here - you can see the differences clearly.
At the top of the guitar is the head, where the tuning pegs hold the strings. Tuning is done by turning the pegs. The strings run down the neck of the guitar and across the fret board, the metal inserts (called frets) are what change the pitch of the string when a finger presses the string down behind them. The strings run across the sound hole in the body of the guitar and this creates the sound. They are fixed to a wooden assembly on the body of the guitar. Classical strings are attached through a knotting process, acoustic strings usually have a metal ball on the end.
The strings are arranged from left to right, lowest (thickest) to highest: E A D G B E . In order to tune the guitar correctly you will need a pitch pipe, electronic tuner or an app on a phone. New strings tend to go out of tune a lot until they settle in. Strings can be replaced individually or as a set. In practice, the upper strings tend to wear out more frequently, so having a spare top E string in the guitar case is a good idea.