I'm sure others will agree with me on this - don't jump right into endgame. GW2's levelling experience isn't a rush to endgame because "that's where the fun starts". It's meant to be enjoyed all the way through, and then enjoyed even more when you reach endgame. This is because of a number of reasons:
Low-level areas aren't irrelevant to high-level players, because high-level players are scaled down to the level of the area. Therefore, you'll run into players from all sorts of level ranges and even big events (such as world bosses) with lots of players taking part.
The game doesn't shove lists of backtracking, fetch, and "kill X of Y in area Z" quests down your throat for you to level up. Levelling up comes naturally from doing anything from exploring, to gathering, to crafting, to dungeons, to story instances, and so on. The only quests in the game are what are known as "renown hearts", which can be completed at your own pace and leisure, and often fill themselves up just by doing nearby dynamic events.
Levelling isn't all that grindy and doesn't take that long.
The game introduces a lot of key concepts to you very progressively throughout the levelling process. You'll start by learning about the basic stuff like dodging and gathering, and gradually get more and more advanced, where it teaches you things like gear slot upgrades, combo fields/finishers, and so on. This is all stuff you'll miss if you jump right into level 80.
My advice would be to start another character which isn't a thief (maybe some other class that you thought looked interesting) and level it up naturally. You'll get a lot of gear and skins along the way, and it'll teach you about the different types of gear rarities, etc. Then, you can always revisit your thief and play it through properly once you've levelled up your other character.