You should not come here unless you have some skills. Things run different in this culture. A simple job in Starbucks or any similar job, pays you the kind of money you need to stay in a room with 2 or three others, because you won't be able to find a room on your own for this salary. At the bottom line you will have some money to buy you food, but not much!
The only chance I see to do this in Qatar is going to a hotel and work in the restaurant or as cleaning staff. The advantage would be that you might be provided an accomodation.
I suggest you first discover how your world (europe) works and thereafter when able to manage life in the culture you know, you start discovering how other cultures work.
My personal opinion is that it is very dangerous considering to come here in your situation. For an 18 year old who wants to learn life and has (as you said yourself) no experience in living alone and managing daily life, Qatar is definetly not the place to learn it!