A few question about Pathfinder.

Greeting, I am fairly new to DnD and after reading the whole core pathfinder book I have a few question : 1) Prestige class : After meeting the requirement to obtain one, does the player that choose the prestige class still gain level in his previous class he was before? If not, than its mean he better wait until he is level 10 before switching to get the maximum out of the 2 right? (Considering level end at 20) 2) As a GM, can I take a published campaign, say its a homebrew, and just use the setting that I want as we go on? 3) Flat footed : If I understand well, this mean that if an enemy have his first turn before me and try to hit me, I loose my dex bonus to AC until it's my turn right? (Unless I have feat and skill that ive read somewhere) 4) If I want to start DM my first game ever (After being a player in a few of course), do I need most of the book, like bestiary and the advanced book, and maybe 1 or 2 published campaign? I think thats all for now, ty for your time! Like the site!

1) No. Not really, you can still choose to advance in your original class or any other class after choosing a prestige class.  Example: I can take levels in the following order if I wish: Fighter for first level, wizard for 5 levels, Eldritch Knight for 5 levels, take more 4 more wizard levels, then take 5 more levels of Eldritch Knight. Total: Fighter1, Wizard9, Eldritch Knight 10. I could have added other classes in there instead.  2) Im not sure what you are asking 3) correct unless you have Uncanny Dodge.  4) Not necessarily. You could use the Paizo PRD.  - Gauss

2) If you are the game master you can mix and match from as many imaginary worlds and campaigns as you would like. In fact I would encourage it.