"My guitar is on the desk."
Translation:Моя гитара на столе.
стол here is in the prepositional case, which used here to indicate a location of the guitar:
my guitar - is where? - on the desk
case singular plural
nominative сто́л столы́
genitive стола́ столо́в
dative столу́ стола́м
accusative сто́л столы́
instrumental столо́м стола́ми
prepositional столе́ стола́х
my guitars - are where? - on the desks
мои гитары - на столах
It helped me to think of it in a different way:
Every form of 'to be' is omitted in Russian, I am a cat is Я кошка, without a 'am'. However, there is a one exception, that is used in a weird way sometimes. Есть means 'there is', in the sense of 'if im speaking very generally, there, somewhere is ...", so you can say things like "Бог есть" (there is a God ==> God exists) or "В доме есть собака" (there is a dog in the house).
(though if you want to say: The dog is there, at a specific place, you say "Там собака", literally "there".
Then, there is the weird Russian way of expressing possession, so instead of saying "I have a cat" they say "To me there is a cat" (У меня есть кошка). Thats just a standard expression of saying it. Apart from this one exception (which is used strangely sometimes) there is no word for 'to be' in russian.