Learning in Imperian is a simple system. In order to learn your skills, you need to seek out a tutor to learn from. This could be another player, or an NPC (non-player-character) tutor.
Learning skills costs lessons. Every new player begins with 150 lessons, and you can gain more in a variety of ways. You gain 5 lessons per level, which means if you completed the introduction tour and quests you will have 175 lessons to start out with. You also gain NeoCredits by leveling, but only at certain levels. A NeoCredit can be converted to 6 lessons. Upon gaining levels 10, 15, 20, and 25, you will receive 5 NeoCredits (30 lessons). Upon gaining levels 30, 35, 40, 45, 50, 55, 60, and 70, you will receive 10 NeoCredits (60 lessons).
You can also buy Credits in-game and from the Imperian website. These differ from NeoCredits in that you can use them to buy artifacts (powerful items) or transfer them to other characters (for free or for gold) in addition to being able to convert them to 6 lessons each.
Additional information can be found in-game, or here on the Imperian website.
Below is a table of approximate lesson and credit costs to advance to another skill level in a Main skill. Mini-skills cost about half as much as the values in the table.
Lesson Cost Calculator
Below is a lesson cost calculator. Simply enter your current skill level, percentage to next skill level (both shown on SKILLS), your desired skill level, and your desired percentage to next skill level. Then, click on the "Calculate" button. This will calculate both the amount of lessons, and the corresponding amount of credits needed in order to reach your desired skill level and percentage.
Note: these values are approximated using the information in the above table.