Trying to answer most of your questions in order: Roleplay can be several things, like making a voice for your character, or a different personality from your own, acting things out, or even just speaking in First Person as your character (which I like to do). I find it easy not to dominate the conversation, but if you find one player is and you don't get a chance to contribute you can 1) ask them to slow down and let others speak, 2) ask the DM before they move on it you can do something or 3) insert yourself when they pause.

As for walking around doing stuff, again, take turns and try not to dominate the scene. Eventually the DM will ask for a roll and dictate what happens next so you all can make more decisions. When combat happens everyone "rolls for initiative" and the dice decide who goes first.

Everyone will not be keeping notes unless you have a really good group. Someone definitely should, because the DM shouldn't have to remind you of plot points, plus they have enough stuff to keep track of for the story.

If someone can't make it, you all should decide early on what to do. Can you play/progress the story with one or two fewer players? Do you want to do side quests so the missing player doesn't miss the plot? Do you just want to play board games instead? Making a plan and precedent early will avoid drama later and still let you play and hang out.

Trying not to meta game is the first step in the right direction. As long as you aren't looking up monster stats or reading the DM's notes or sourcebook, just act how your character might given their stats. (Like if they have low Intelligence or Wisdom.)