Your class is what your character does. It is the first choice worth making, because it decides more of the sheet than anything else you pick — and most of what it decides, it decides for you, the moment you choose it.
There are eighteen classes to read. This page is about what you are actually choosing between.
Each class names the ability it runs on — the one your attacks, your spells or your signature features key off. It is not a requirement and nothing stops you ignoring it, but a class played away from its primary ability is a character that misses more often than the table expects.
That single number is also the strongest link between this page and the next two: the class tells you which ability you want highest, and ability scores is where you make that true.
Every class chooses its subclass at 3rd level, without exception. A subclass is a set of extra features layered onto the class — the same chassis, pointed somewhere specific.
If you are building a 1st-level character, you do not pick one yet. It is still worth reading the options for the class you are considering, because they are where a class's range actually shows: two characters of the same class and different subclasses can play nothing alike.