If you are a beginner and never had guitar lessons or played before is better to start with a classical or electric guitar because the strings of the acoustic guitar are tighter so it's more difficult to play. In the case that you have an acoustic guitar, you can always change the strings (as is shown before) and put some special "low tension" strings.
If you are going to start to play the electric guitar you just need a 10 or 15 watts amplifier to play in a flat or apartment, and its better if the guitar is not the cheapest. In the future if you learn and have some level then may be you would play in a band with other friends and then you would need a bigger amplifier but you could still use the same guitar.
There are different sizes of guitars in all (classical, electric, acoustic, etc...). Apart from the standard size, there are smaller sizes for children of all ages. In the case of the classical guitar those smaller models are called "cadete" or "requinto". Other times fractions are used to identify the different sizes o0f these models like 3/4 instead of 4/4 that is the normal size.
Having in mind children and young people comfort, it's good to consider the guitar's weight in particular if it's an electric guitar or bass. Considering that the first guitar is usually not a top range guitar, it's just a guitar to start learning, it would be good to focus the attention on comfort. When a guitar is heavy (may be because is of good quality and made in good wood) and if the student is not an adult that weight could cause fatigue and become a reason for not practicing.
AccessoriesIt's good to have an extra set of strings (just in case one string of your guitar breaks), some extra picks, an extra cable and small accessories just not to have the inconvenience of going to the store again for buying them.
If the guitar is not going to be moved from home you just need to buy a gib-bag or you could have a stand (for the ground) or a wall holder. For young students it's easier to have their guitar on those kind of stands so when they are going to practice they don't have to take the guitar out of the bag only take it from the stand. If you are going to take your guitar out of the house or in a trip (like a holiday trip for example) there are padded bags (that save from bumps) or cases. Most commonly, people buy a gigbag or a case deppending on the price of the guitar they have bought and how often they are going to move it from home.
There are some products to clean the sweat of the fingers especially in summer. If you have an electric or acoustic guitar that would extend the usuable life of the strings. Otherwise they could oxidise with the sweat and break. There are also products for avoid that the neck gets dry (they can be useful depending on the weather of the place where you live), although many times when you buy a guitar comes with some little bags with some product inside for exactly the opposite: dry the neck and avoid it has humidity.
There are some other accessories like the pedals effects like distortion, flager, chorus...or multi effects. These pedals are used by the professional guitar players so are not necessary to start learning since most of guitar amplifiers have effects (like distortion) although is more uncomfortable to stop playing and press a button in the amplifier.
Normally to conect a pedal effect it's necessary two guitar cables. One of them goes from the guitar to the pedal and goes to "input" plug of the pedal and the other cable must be connected in the pedal in "output" and to the amp. Once you have bought your guitar you need to to take care of it.
Miguel Angel González Aguado has a 30 years long History of Concerts. In this website you can have all the information about him like for example his academic degrees from Conservatoire and University, the guitar lessons that he offers and the places where you can listen to his records.