On a given spell or attack, there are DCs or "difficulty classes". These are rolls the players have to make to avoid or take reduced damage. Most often (though not always) spells lay out what this magic number is, the standard is (10+spell level+casting stat mod)

Spell Example: a sorcerer with 18 Cha casting fireball. Anyone caught in the fireball gets a reflex save for half damage. The Difficulty class of this save is 17 (10+ 3 for a 3rd level spell + 4 for the charisma bonus). There are feats, traits, etc that make DCS harder to beat but we'll leave that there for now.

The other DCs you'll see are on skill checks(perception, survival, acrobatics, umd, etc) those are a little trickier. The DM decides how difficult these should be for the players and sets the DC accordingly. Level 4 PCs with +8 in acrobatics should have no problem with a DC 15 check to jump over a moving cart. In the same wheelhouse, you should make sure your hard to beat checks are hard to beat. A heavily fortified door with guards on the other side should require a higher than normal umd or stealth check(DC 25-30) to break in.

I dont have links right now, but the d20pfsrd has everything you could possibly beed as a new dm. Hope this helps.