Originally Posted by gRomoZeka
Have you heard about declension?
In Russian nouns, adjectives, numerals etc. change their endings, depending on what case and number they are in (sometimes gender also matters).
So "a" that is added sometimes is a part of that system. (But "ты хочешь пойти в ресторанa?" is wrong, it should be "пойти в ресторан_"). You need to know more about the case system to understand Russian, it's crucial and is equivalent of the word order in English.
A couple of examples ("ресторан" and "Путин" in different cases):
ресторан, Путин
ресторана, Путина
ресторану, Путину,
рестораном, Путиным
ресторане, Путине