One of the quickest ways right now to level from 1-100, on any class, is to quest your way up. Heirloom gear, with the inherent bonus to experience gains, will make this even faster. Heirloom gear can be bought for a fair sum of wow gold now. If you choose to upgrade in this way, I suggest you that Mmogah is a good site to buy wow gold(wow gold kaufen). There is an article which is named Is It Safe to Buy WoW Gold at Mmogah. You can see the details of it.
Many people, however, prefer to level up using the Looking For Group finder (LFG), and run dungeons. This really only is faster than questing if you're a tank, which requires foreknowledge of most dungeons to perform adequately; or if you have never leveled a character in WoW at all.
Leveling through questing
Questing: Leveling through questing is one of the better ways, especially for new comers, to experience the content that has come before the current expansion. It is also a very rapid way to gain levels, since the experience points awarded for quests have been increased in older content, in order to speed the leveling process to such high levels as is required now to appreciate current content.
Most quests are very easy to complete, and if you're good at planning a route around a small area within a zone, they can be completed very efficiently. This is best accomplished by using the Quest Log and the game's Map in order to see what objectives are where for the quests you intend to complete, and how you can make a quick path. Often, objectives overlap within an area, so by killing the targets within the area the Map shows the items can be found you'll get the drops you need to complete each objective, or clear out enough area to pick up items that may be laying on the ground safely.
A little bit of experience in this is very valuable to develop a flow of your questing style, and become very efficient in gaining experience through questing. You may also benefit greatly by continually improving your gear through quest rewards, at least in the lower levels.
Dungeons (LFG) you may also level fairly rapidly by using the LFG tool in order to complete dungeons, which reward a substantial amount of XP upon killing the final bosses of each. This is best for those who are willing to tank in these dungeons, since queue times are practically nonexistent for tanks in the LFG environment. Most times I've queued up as a tank in LFG, I've had to wait a second or three before getting the notification that my group is ready. If you are playing as a damaging role in a dungeon, you may have to wait for up to 30 minutes (usually at higher levels) in order to be grouped into an actual dungeon.
However, if you're a newer player, especially one who has never been in these dungeons, you're probably not going to be able to tank very well. It requires, at a bare minimum, knowing where to go in the dungeons, since the tank is invariably expected to be experienced and to lead the group to each objective. I'd also like to point out here that if you're too new to the MMO genre to know what a tank is, you definitely should not be tanking. This is a role that requires you to know what you're doing in order for the members of your group to survive in dungeons long enough to finish and complete quests or objectives within the dungeons.
Healing in dungeons is also a role that at least requires you to know how to heal a fair bit. It's not quite as big of a responsibility as tanking, but certainly not something most people can jump into without at least a little practice and familiarity with healing spells and their usage. An added benefit for healers in LFG is that your queue times are usually much lower than for a damage role, but usually longer than that of a tank.
Use another higher level character
The other way to quickly level a character up requires another account with higher-level character on it, whether a separate account you own, or the account of a friend who is willing to help you out. The faster method here is to take the bigger characters from the other account, group them with your character you want to level rapidly, and go into dungeons appropriate for your level. You use the powerful, leveled character to quickly wipe out all the targets in the dungeon, and complete your quests for additional experience. If you're used to doing this, you can quickly rack up some experience without having to wait for queue times. It can feel a little mindless to tag along behind "big brother or sister" and just pick up loot (which the higher-level characters often have little to no use for). Also, you have to either enlist the aid of a patient person to be your muscle, or run two instances of the game yourself, either on the same PC or multiple units. This can be a mildly daunting enterprise, especially since you have to continually switch back and forth between the higher and lower characters, to move the inactive one along, loot, pick up quest items.
These two options above are all the good ways to level up quickly. Hope it can help you.