Here are some tips to increase your exp gain and help you level faster.
1) Playing as a Human gives you a passive exp boost. Their racial passive abillity Hostile Investigator increases the Experience you gain from killing monsters by 5%
When you begin the game the first major town you visit is called "Silence". In this area several quests will give you diplomas. You may also receive them from the RNG treasure chest in the top left corner of your screen when it is available. There is a small island in the lake just to the east of Silence that has an npc named Marc with a purple quest available that can be done daily to earn a diploma. His quest is to simply hit one of the training dummies near him. These diplomas can be traded to the Legacy vendors Vitak and Nieli near the dock on that same island for gear. You can purchase weapons and trinkets by turning in diplomas. These legacy gear pieces each give a bonus 15% exp boost that stacks with each other (1 weapon+1 trinket). Note, having multiple exp boost weapons does not stack. The minimum level requirement for these items is level 1. As you level up you can trade in your legacy weapons for more powerful ones. To purchase the upgraded versions you will need the appropriate number of diplomas plus the previous tier weapon.