Technically, 484 is just the definition of theft. The various levels and theories of theft are listed in the other sections. They're often written together to designate you're charged with theft - but particularly petty theft.
Neither is an infraction. Petty theft is theft of anything under $950 in value (with a few exceptions). It is a misdemeanor that carries up to 6 months in county jail and/or a thousand dollar fine.
There is a section where theft of under $50 from a retailer can be an infraction, but that charging/reduction decision is up to the prosecutor. Any theft - from a penny up to $950 could be charged as the misdemeanor.
Penal Code section 484 is the general theft heading. There are many degrees and specific theft sections of the Penal Code which are under this general heading of PC 484. Petty theft, Penal Code section 488, is one of the lesser degrees of theft. PC 488 may be charged as either a misdemeanor or an infraction.
To add to the two accurate answers above -- you need to consult with a lawyer. If you are charged with a misdemeanor, you want to hire the best criminal defense lawyer available. if you cannot afford to hire a lawyer, you can apply for the services of the Public Defender. Theft is a moral turpitude offense and, if it gets on your adult rap sheet, it can follow you around your entire life and make things difficult.
If you are a juvenile, the Public defender will automatically be appointed for you (although you or your parents could still hire a lawyer if you want). As a juvenile, even if a petition were sustained, you could have your record sealed after you turn 18.
Either way, talk to a lawyer about how to avoid having an actual moral turpitude misdemeanor on your record. Good luck!