I can't seem to reach the stickers in the level where the hook is introduced, I know you're supposed to swing high enough to get there, but I can't seem to do it. Any tips?

If you get an old lady's keys, RUN. Forget your keys, it's not worth it. 
 
 
RUN.

I had trouble as well, im sure everyone did.  I found that I made it the highest when I threw my sackboy far away from the thing you grapple to.   So just swing, let go (throwing yourself to one side), grapple back to it when you feel close to where grapple range ends and swing back the opposite way.  Do that a few times to gain momentum, then let go as soon as you start to swing up, it should be enough to send you flying to the items.  It may take time and get frustrating, but you will eventually get it!!!  Trust me, there are WAY more frustrating moments in LBP2 :P

Just skip them. By the end of the game you will have mastered the grappling hook and you'll get them first try.