Go out and explore the world, and levels will come all by themselves. You get experience for everything you do: uncovering new areas on the map, discovering points of interest, viewing vistas, beating skill challenges, even gathering ore, wood and plants will give you experience. Then there is participation in dynamic events, doing daily achievements, and of course killing things, but the last one is really only a small part of your experience gain.
Do try to stay in areas that are not (much) above your level. Enemies 4-5 levels above you are hard even for experienced players, and quickly become impossible if you don’t know the combat system, your weapon skills, traits and utilities (or don’t even have access to them yet). You’ll get just as much experience if you stay in areas your level or even below (in which case you’ll automatically be down-leveled to keep things interesting).
Exploration experience (pois, vistas and the like) does not depend on the area level at all, and while kill experience will decrease slightly if you are down-leveled a lot, it is really not noticable in my experience. Try to participate in all events you come across, and explore areas as thoroughly as you can, and you will find yourself at level 80 before you have even seen a third of the world.
If you absolutely must speed up your leveling, get some consumable buffs. Cooked food and utility potions can give you +10% kill experience aside from other buffs, which will increase your experience a bit. You will also gain a one-time experience boost for one of the early personal story chapters I think.
Be aware though, that by quickly leveling to 80 you may miss out on some vital experiences with your class and utilities that might help you enjoying the game once you have reached max level. The combat system and game in general is considerably different from most other games out there, and you may as well take the leveling time to getting used to it.