see also: Grinding Guide
In order to optimize your leveling, you want to get to Heffner Camp as soon as possible. Most people will have a "transport Heffner Camp" option that they can use to get there, you may have to get your second class (in Varanas, once you reach level 10) or third class (in Varanas, once both your classes are level 20) before you get this transport skill.
Make sure you do your 10 daily quests every day. If you don't have the time, you can buy the quest items on the auction house, and hand them in. Once you have two classes, and one is a higher level than the other, you can accept the higher level quests with one class, and hand them in with the lower level class (up to a level difference of five levels).
Beginning Area - Your main job here is really to accept all quests in the area and complete them one at a time but i would rather grab a whole bunch and do all of them then hand all of them in because you might find that many of the quests you have been doing are only a stones throw away from another.
Try to get to Varanas City once you're level 10, so you can acquire your second class and start leveling it. Both from the Elven Island and from Logar there are ways to transport to Varanas in Silverspring. You can add classes (up to two, for a total of three) in the class hall.
The warden/druid are on the slope where you come down after coming to the mainland from the elves beginning area, this is located far left of the map near the bottom.
Silverspring (First Area) - The quests here are straight forward. They don't pose much of a challenge if you are 15+ bar a few, like the experimental wolf and the twilight mine. Your level once finished in this area will roughly be around 25+. This area contains the start of two epic quest lines (marked in purple instead of yellow). You should pick these up and follow them as they have an enjoyable storyline and unlock various areas.
Aslan Valley (Second Area) - This area can be used to get to level 30. However, only go here after you run out in the Heffner Camp area below. The area has some hard monsters like Fire Mane Boars and Wandering Ents. Beware of the experimental dragonfly.
Heffner Camp (Third Area) - Right this is where the real fun starts this area has high XP from quests all through the Coast Of Opportunity and the zone that follows, Xaviera. The quests are set up in a way that you can follow the entire valley. It also provides "Old Bag" quests, which allow you to get all your elite skills without collecting the materials that you'd overwise have to collect elsewhere. It is especially the daily quests in this area that dwarf the XP compared to daily quests elsewhere, so ideally you do all your daily quests here.
If you are finding it hard to get your sub class to the same levels: there is a daily quest called Life Hormone outside of the Desert Investigation Post in the small post just across from it that gives 61,555 xp, which is to collect 10 samples of the Aoluona from the Burrow Food Plunderer. They might be a bit hard to take down and annoying to farm 100 for the 10 daily quests you can do but if you take your time and don't rush the drop rate of Aoluona is high so you will get it from almost all dead Burrow Food Pluderers and they rack up quickly, also doing this daily quest will get you nice TP and 2541 Gold.
Xaviera - The area after Heffner adds an even bigger bonus: the old bag quests here give you a 20% XP buff that also counts towards quests you hand in. So if you alternate these with the daily quests this means your daily quests will get an additional 20% XP. The buff lasts only 10 minutes, but can be renewed by handing in another old bag quest. This area also contains the famous "butterfly daily" for level 48+, which requires you to kill 5 butterflies for over 266,000 XP. There is almost always a group killing them that you can join, which makes getting the kills very fast. There are also other dailies that provide this much XP, but there are none that can be repeated in one day that give more.
Ancient Kingdom of Rorazan - At level 69 head to AKR where you can now do the quest "Extinguish More Flames" quest which is at the intersection near the Snoop. If you are far along your questline you will also have access to the "Demonstration Battle" daily which is in the City to the East of the Snoop. Keep to the very left as you enter the City. A nice feature of the new zones is that the Energy of Justice quests mirror the daily quests, so if you know how to do one, the other is exactly the same. Expect 297k XP from dailies in this zone.
Chrysalia - At level 71 you can accept the quests in this zone for 300k XP. Popular quests include "Arrow Heads" requiring you to kills bugs near the Kulech Bones Nest and hand in 10 arrow heads/daily.
Merdhin Tundra - At level 75 you can accept quests in this zone for 333k XP. The zone is so new that many of the dailies are as yet not compared.