By default, your XP bar is displayed below the HP/MP/TP bars. You can also see progress bars for each of your individual classes in the Classes tab of the Character screen. Edit: If you mean on the lodestone, look at the list of classes under your main stats. You're currently 225/300 on LNC.

If you or your party claim a mob (its name and health bar are red), you get full XP, quest and regular items it drops. If you do 30% or more damage to someone else's mob (purple name and health bar), you get full XP and quest items it drops, but not the regular drops. So it's possible for up to 4 players/parties to each get full XP from one enemy (player/party 1 claims, 2-4 each do 30% of the damage).

There are two account levels. The cheaper one allows 1 character per server, up to a maximum of 8 total. The more expensive one allows up to 8 characters per server, up to a maximum of 40 total.