You almost never actually use the gear that is rewarded to you or dropped by monsters. You will need to sell or salvage them in order to craft or buy the specific exotic gear you actually need, and buying it is a lot easier.
You can either buy it on the trading post or using dungeon tokens. If you use the trading post, make sure you buy a level 80 item with the exact stat combo you want. Start with weapons, chest armor, and leggings since they contribute most to the total. Don’t forget about trinkets, but they are not available in as wide a variety of stat combos.
If you decide to use dungeon tokens, you will need to research the dungeon(s) that offers the stat combo you need. Citadel of Flame path 1 and 2 are popular choices because they offer Berserker items and are easy to do (with practice). Arah takes forever and is very unpopular.
Unlock explorable mode by playing story mode just once. Then learn how to survive explorable mode, either by reading or asking. There are usually a few tricks to each one. Most of them emphasize melee fights, which encourages stacking (standing in the same spot) so you can share boons and be quickly ressed when you die.
You can get a complete set of exotic armor plus one exotic weapon for <1800 tokens. Each dungeon gives you at least 100 tokens the first time you complete it each day.