For Beginners, deciding which form to go is simple:

If enemy team has 3+ ranged characters, go blue form

If enemy team has 3+ melee characters, go red form

Something to look at for more advanced players is:

Is the enemy team comp mainly squishy? or tanky?

Squishy: Blue Form Tanky: Red Form

Something else to look at:

Does your team need a fighter/frontline? If yes, go red form

Does your team need an ***asain? If yes, go blue form

FFO, this is what the rank 1 Kayn does. FFO stands for: Farm, Form, Objective. So if your team is winning, play for objectives. Kayn is a great way to get into jungling, playing him will really teach you how to find a balance between farming and ganking, with Kayn you want to be powerfarming while at the same time ganking lanes to get orbs for your form. As Kayn, you also want to be invading the enemy jungle a lot. You will also want to be playing a bit more safer until you have form or level 6.