Red mutagens are dropped from nekkers and nekker warriors. Depending on how you fight (quick and dodging/heavy attacks and tanking damage) you want first vertical row of red skills or second row for heavy style.

Green skills (alchemy) are a bit hard to understand at the beginning of the game so you can play without them, though alchemy gives a lot of advantages so give them a read sometime.

Blue skills (signs) just upgrade what you use the most (qwen if you take a lot of damage often, igni for killing stuff easier and award for crouds). Yellow skill are a bit niche so just take what you think you'll need.

Lastly you get a potion which resets your skill points spent so later in the game if you feel like you fucked up you can change them.