You can find skills and other upgrades by selecting a mission, then select you character. When you get to the "Prepare For Departure" screen click on "Growth Map". Skill islands have Stars on them. You only need to put 1 point into each skill. It doesn’t need to be maxed but if you want another achievement then I would recommend maxing 1 skill to level 3. It doesn’t need to be for a single person so you can get a skill for every character if you want.