When you start playing you will notice a special GUMP that looks like a wax seal (below)
This is the level system GUMP. Here you can access many options to add information as you gain levels from experience gains. From this screen you can add to your Strength, Dexterity, Hits, Stamina and Mana. When you hit level 10, you will be given a Profession Point which will allow you to choose a profession to gain special advantages depending on what you choose. When you pick the Mage Profession the Intelligence Statistic is unlocked so you can then add points to your intelligence.