XP is gained from killing enemy units, and can be increased with a skill and a ring.
When you level up you are presented with 3 skills to level up, randomly chosen from a set of 23 skills. The game will not present skills you have already maxed out as options.
The maximum level is 200.
The total number of skill points is 199, as discussed on the skills page, so at maximum level you will have all skills at maximum.