Experience is gained from killing mobs. Although all mobs will give you some experience it is best to kill mobs near your level.

Skill points are used to learn your classes unique skills. Each class would require 100 SP in order to learn all of its skills but in game you can only obtain a maximum of 90 SP. This forces you to choose the skill build of your choice. No need to worry about investing in the wrong skills, however, you will have a chance to reset your skills and replace all your SP once you're level 25

SP is gained every level and the amount of SP depends on the level. Here is the amount of SP gained at each level starting at level 2 and ending at level 25: