Preparing for a campaign depends heavily on the system, the GM, and the specific campaign. I would definitely recommend at least being familiar with the most commonly-encountered rules for the system you'll be playing, and having a familiarity with the setting if it isn't a custom one created by the GM. Beyond that, you should ask the GM what he or she expects of you. Mobile access to the tabletop is a perk of being a subscriber (either Supporter or Mentor), although I think you can access a campaign from your mobile device if the campaign owner is a Supporter, even if you aren't. However, I would generally recommend against using a phone to play, because you would have very very little screen real-estate to utilize. I have a 10.5" tablet, and I still feel it's too small for being the GM, although it's fine for being a player. That's just personal opinion, though.
