Two cents. It's all a lot of questions - it's more for you to think about and somewhat figure out yourself since there is no one size fits all.
On a basic level you can look at pocket as leading the charge - capitalising on advantages that other players on your team have created. Be it damage or positioning, to something like having an ammo advantage - you are mainly the one pushing forward dealing out the damage once this advantage has been created.
There are no real "tips" to positioning - you have to look at the current situation and adjust accordingly: do you need to be on high ground, do you need to have more players on flank, do you 100% need a scout with your combo? These are just some of multiple questions that you need to think about when playing, and it just generally boils down to what is happening.
One of the most important things to do is have good communication with your demo. Quite often you and your demoman can play simple games of bait and switch - where one takes the damage/wastes other teams ammo, then the other follows through after that to exploit the small window of the other team not being ready/no ammo to deal larger amounts of damage and take positioning.
On the basic level as pocket you just sit on the medic. Later on you don't want to be attached via an umbilical cord as quite often, focusing on sitting on your medic makes it a lot harder for you to take advantage of extremely small advantages that your team have created for you. You want to look out for your medic, sure, but if you have killed half their team because you "left your medic" and he dies (but you in turn, win the round/point/rollout), then wasn't that a small sacrifice?
Watch povs, stvs - try and figure out why the soldiers are doing what they are doing. Some soldier may be doing a dumb amount of jumps - why? Could he be jumping around like an idiot because it's fun? Is he trying to get somewhere fast? Is he creating a distraction for his team?
tl;dr - play more, watch other people play, think about the game, shoot the bad guy.