In this post I will explain the best ways I have learned to level in world of warcraft. These are from my personal experience.
The most fun way to level (for me) is questing in a group. I suggest getting a free guide like jame's leveling guide to help you be the most efficient. There are many paid guides you can buy as well. most of these guides boast lightning fast leveling up to max level. While after a couple of characters this is probably true, for the average player these guides are great to follow, but it doesn't make leveling exponentially faster like the guides claim. However, these guides are great if you want to get quest completion achievements. in conclusion, these guides are great for seasoned players to level faster through questing, but for new players and players that only play a few hours a week, these guides are money spent without worthy results.
One of my other favorite ways to level, especially when I am listening to some hard rock and screamo music is to just KILL KILL KILL. Grinding is a simple way to level. It takes a little longer to level than questing, but man, it sure does relieve stress in a weird way. while it can be done by any class and spec, some are better equipped than others. The best class to aoe (area of effect) grind used to be the frost mage with their massive aoe damage and their ability to get away from melee mobs and freeze them in place. They are still pretty good, but my favorite aoe grinder is the protection paladin. They have so many damage mitigating abilities, they wear plate mail, they have a shield, PLUS, they can heal themselves easily. A decently geared protection paladin can pull about 5 mobs at a time and kill them quickly with their aoe abilities. pulling a single mob at a time is another way to grind. the two best equipped classes are rogues and feral cat druids. Rogues can kill extremely fast and have the ability to vanish if things get hairy. However, druids can also heal themselves which is worth mentioning....
Dungeons are a great way to level if you are a tank or a healer. But, if you have 4 friends that can quickly finish dungeons, this is a wonderful way to get tons of experience while also getting great gear. This can get boring if you constantly get the same dungeon through the looking for group tool.
PvP is a fun way to level, for some. But unless your faction is winning, you will not get much experience. However, it is worth noting that in one afternoon of playing in battle grounds, I got a quarter level per alterac valley battle during the holiday weekend. The battlegrounds Holiday weekends are the best time to level through pvp because there are tons of people playing them all day and night. the longest I had to wait was 2 minutes for a battleground que.
Another way to level (albeit slowly) while making oodles of money is to level up certain professions. The two professions that can help you level are mining and herbalism. As someone that has leveled tons of characters in many different ways, I would be remiss if I didn't mention that if you are playing on a recruit a friend account, if you are near your triple experience partner while leveling, you also get triple the experience from picking herbs and mining as well, given you are close enough to your friend.
These are the most common ways to level in world of warcraft! Happy Leveling :D