Pacifist route:

In the pacifist route (no killing route) you want to ACT on monsters, until you manage to make them not want to fight you (their name will be yellow), and once you do that, you give them MERCY and SPARE them. Boss fights work a bit differently, MOST of them just require you to survive, with the dialogue changing until you make it to the end of the fight. For the Asgore fight, ACTing will not work, and you will be forced to FIGHT him. You will not have to kill him, don't worry but you must fight him. If I have missed any info please comment on it

Genocide route:

Kill everything. In the ruins, you will have to keep killing every monster, until the save points say determination. THEN you do the next boss fight, and continue killing everything in each area, if you miss an enemy in an area, but go and beat the boss, you will be off the genocide route. If I have missed anything please tell me.