The three skill categories are Seeker (blue), Warrior (orange), and Scavenger (green). Each category has 4 or 5 skills that are unlocked through story progression, 3 that are rewards for completing challenge tombs, and 12 that are unlocked by applying skill points at base camps. The total number of points needed to unlock all of the skills in each category are 23 for Seeker, 20 for Warrior, and 24 for Scavenger.
Each category has at least one skill that won't be unlocked until you've completed about 3/4 of the campaign, and by that time you will have earned enough points to unlock maybe 20-25 of the elective skills, so for the first half of the game, don't worry about dumping all of your points into one category; you should probably go for a balanced build and at least acquire all of the 1-point skills in each category. Once you get into the second half of the game (i.e. reach the Hidden City), you should start focusing about half of your skill points on one category of the three. I chose Scavenger because it seemed like all of its skills were useful. If you don't rely on Survival Instincts, many of the Seeker skills can't help you, and so many of the Warrior skills are bow-specific. But if focusing on Seeker or Warrior fits your playstyle better, do that.
EDIT: HolyHalfDead comments that Seeker and Scavenger can be completed once you reach the Hidden City, but Warrior cannot, so if you're in a hurry for some reason, don't go for Warrior.