How to punch someone

If you mean the gameplay tutorials, you can get a basic idea of that through the earliest levels, The Gardens, which will go over all the stuff like jumping/gripping/etc. If you mean the creation tutorials, if you hit Square to go to the Poppit menu, then move to the Tools Bag, then hit Square instead of X in the Tools Bag, you'll see the text from the tutorials you've finished already. If it's blank, then you still need to do the tutorial, which you then should be able to do by hitting the ? mark.

Just go into the level you want to play and than wait for other to join (You have to stay near the entrance of the level though. If you go past the sackbody sign the game will close it. Be patient and someone will pop-out. (Hopefully a skilled player) Or you can send invitation to your friends while waiting for them in the level. (Send them a message with the name of the level). Have fun :)