You have two methods
1: do everything. Do all quests, dungeons, delves, discover all Locations and kill everything you come across.
2: find an area that has a lot of enemy NPC's, kill them all, move to another area close by and kill them all. Try to create a loop so when you get back to the first area the enemies have respawned. This is called grinding and there are plenty of known areas to do this. Search the forums with the term "grind spot".