The fastest way to level up is to defeat monsters that are near your level or a little higher for your level. Don't fight monsters that you'll have some "Missed" hits on and don't fight weak mobs for a long time (that takes longer). Fight ones that are just right. Be sure you have enough potions as well, you won't want to die and lose precious Experience Points, would you?
Also, Party Quests (PQ's) are fun and are EXP-gaining, but sometimes, it's hard to find one at your current level due to the stages of the Party Quests. If you feel like doing Party Quests, do it at your own risk and be sure you know what you're doing.
If you feel like spending money on PayPal, then do it the guilt-free way by Power-Leveling your character to a certain level. As you get higher into the levels you want them to level you up for, the more expensive it gets and the higher the level your character is at, the more expensive it will be to level it up, even if it's for 1 level (like from level 29 to level 30 is a lot). I do not recommend this because you may be banned from the game if you are caught.