My question is about how much and what spells I can use, I'm new to Pathfinder and unclear on a few points about how much I can use spells, for example, say I can cast 4 level 0 spells a day and I can prepare 4 level 0 spells (might not be book accurate, its just an example), say I prepare like ray of frost and three other spells for the day and I use ray of frost in combat, would I be able to use it again that day in combat, or would I have to prepare it in two of my four slots when I prepare my spells for the day to be able to cast it twice a day?
Wizards can prepare a number of cantrips, or 0-level spells, each day, as noted on Table: Wizard under “Spells per Day.” These spells are cast like any other spell, but they are not expended when cast and may be used again. A wizard can prepare a cantrip from an opposition school, but it uses up two of his available slots (see below).
(Source: Wizard, from d20 Pathfinder SRD.)
In Pathfinder, unlike in D&D 3.5, zeroth-level spells (or cantrips, for sake of simplicity) are not expended by using it. Thus, you have unlimited usage for four (or three, if you are at level 1) kinds of spells you can prepare. You don't have to prepare ray of frost twice to use it twice.
Of course, if you have barred the school of the spell you are preparing, that spell counts as two-slot-worth as you normally do. For example, if you have barred Evocation, you are left with only two cantrips remaining, instead of three, if you prepare ray of frost.
However, leveled spells are different. Obviously, spells with positive levels can only be used the number of times you have prepared. For example, if you have prepared magic missile twice, you can only use it twice, as you have thought initially. The universal rule of using two slots for one spell from your barred school applies normally.
No. That's a 3.5e thing, not a Pathfinder thing. You still prepare cantrips in Pathfinder, but they don't expend the slot when you use them. Because of this, you can cast cantrips at will, though only those which you actually have prepared. You can switch them out whenever you prepare spells.