I´m trying to create a multiclass character for an upcoming campaign with my friends. She´s going to be a human fighter/sorcerer at level one for each class. I have played only singular class builds before and now delving into multiclassing for the first time.
Is multiclassing meant to stop the progress of the earlier class you had before multiclassing or do they both progress at a slower rate or how does it work?
I´d like to be able to progress both classes, but do not know if it´s possible within pathfinder rules.
Calethos gave a really great and succinct answer. I would add just to be explicit, if you are a fighter 1, and sorcerer 1; when you move up to 3rd level you can go back and choose fighter again. So in a way you can progress both at the same time by alternating which class you choose to level in. It also doesn't have to be every other level, you could do 2-1-3-1 or whatever. You can also take 1 level in 20 different classes. (Which, aside from saves would make for a pretty terrible character, so probably don't do that :)
I strongly suggest you look at the optional rules for fractional base bonuses from Pathfinder Unchained and ask your GM to allow them. Multiclassing is already something of a powerdown by design, but on top of that you can really be bitten by anomalies in the math used to generate the BAB and saving throw tables. Multiclass characters tend to end up with one or move saves or a BAB that is too low for their total character level.