Skill points max out at twenty and, especially early on, you will have to spend a ton of those in order to improve the stats of your fighters. So the first thing to do as soon as you log in to the game is to upgrade the Skills. By the time you're done with everything else, you'll have several more skill points to spend. Always do your best to keep the skills of your Heroes (main Fighters in your team especially) at maximum level in order to have them as strong as possible.