Hi JC, welcome to Roll20, Attacking an NPC works just like it does on a real life table top, declare your attack and the GM will tell you what happens based on your rolls. The exception to this is if the game has certain API scripts running in it that may automatically apply damage and decide if the attack was successful or not. You can also make macros that will automatically compare your attack to the targets ac if you want to do fancy macro magic. Id recommend running through the tutorial as it has several good lessons. Pretty much everything else is on the wiki , unless you need help with designing a specific macro/script. There is a section on rolling dice for info on all the parts and commands for the dice roll engine. That should cover your general questions, but do you have any specific questions? EDIT:added link to the tutorial.
