Leveling a character

You gain experience by killing mobs. If those mobs are higher level than you, you get an XP bonus, if they're lower level than you get an XP penalty. You can determine the approximate level of a mob through the command CONSIDER.

Grouping will give you an experience bonus as well.

Over time, if you keep killing the same things over and over, they will be worth less experience to you. This is done to encourage characters to explore the world. Each level you gain will take an ever increasing # of XP. See the below tables for XP needed for the levels 1-75 and beyond, along with their according ranks.