Conditioner
Shampoo opens the hair cuticle, a conditioner closes it again. That's the whole story: less tangling, more shine and less breakage when combing through. Apply from mid-lengths to the ends and keep it away from the scalp, otherwise your hair will quickly become flat. Leave on for one to three minutes before rinsing.
Rinsing out or leaving in is a bigger choice than which brand. A regular conditioner you rinse out. A leave-in stays in and does something different: it protects and retains moisture throughout the day. They are not interchangeable — you can use the rinse-out one first and then apply a leave-in to towel-dried hair.
Fine hair and conditioner can go together, provided you keep it away from the scalp and leave it on for a short time. If your hair still feels flat, choose a volume conditioner or use it only on the ends. Skipping it is rarely the solution, because then it gets tangled and breaks more.
Rinsing with cold water is not a myth, but the effect is small: the cuticle contracts slightly and that gives some shine. Nice if you can keep it up, no disaster if you skip it.