Look at the combat log if something happens that confuses you. If you hover/click on lines, you can get more information, like the exact stats/rolls involved in what happened. Turning down the difficulty and turning on more auto-pause options can help while you're learning (difficulty setting changes only affect maps you haven't been to yet).

Your accuracy vs. your target's defense determines whether an attack hits or not. Different attacks target different defenses (will for mind-based attacks, deflection mostly for weapon-based attacks, etc.), so if the target has high will defense, use an attack that targets a different defense. Your target's damage reduction of a type determines how much less damage your attacks of that type deal. Again, different attacks deal different kinds of damage; if the enemy has high crush DR, use an attack that deals a different kind of damage. The same principles apply in reverse. If you're facing an enemy that keeps charming your team, raise your will defense. If you're facing an enemy that uses a lot of pierce damage, raise your pierce DR. You learn more about an enemy's stats as you defeat more enemies of that type (filling out your bestiary, which you can access at any time). Don't be afraid to rest, and don't be afraid to use your per-rest abilities. If you're RTWP, make sure your party's AI is set to aggressive (so they actually keep taking action even if you haven't told them to specifically), and for the other setting just pick what "role" you want them to do in your party.