Your wallet can be accessed by going to the top left and clicking on the Wallet button.
Treva is a currency that can be obtained by going into the PvP maps (the maps on the top-left of the map) and killing elite monsters. These elite monsters can be found by looking at your minimap and should be indicated by a small red dot. If you don't see any, you can start killing random monsters or change channels. You can quickly change channels with the /nextchannel, /backchannel, or /movechannel number_here commands (type /? to see the commands). Killing these elite monsters give you a chance at creating a portal to the Twisted Pocket Realm. These portals last for a few seconds, and once you go through, you will be teleported to one of the different Twisted Pocket Realm "minigames" (killing a boss, killing rainbow slimes, killing chests, killing mobs from an ambush, destroying towers). In the killing of rainbow slimes and chests, you will get Treva for each one you kill, and completing the other game modes will result in 4 chests to spawn at the end that drop Treva and mesos when opened. These chests have a chance to be golden, which will give you more Treva than normal. You can spend Treva by going to Queenstown and finding the shadow merchants at the top right. One of the vendors offers a glove that gives you 5.4% physical/magic piercing, which is a great placeholder until you get something better. You can also spend your Treva on Gem Dust boxes or Tier 1 gemstone boxes.
Personally I found it to be most efficient to farm Treva with another player. Although you will want to split your rewards, it is much faster since the elite monsters will sometimes have shields that switch between physical resistant to magic resistant.
Rue is obtained through the “Hoard Things” daily missions (each mission gives 4 Rue) or by doing the World Boss Daily Missions provided by the NPC (Aliyar) in Queenstown (each World Boss Daily Mission gives 1 Rue and 5 Blue/Red stars). Rue can be spent by going to the vendor in Queenstown. The most notable items that you can buy from this vendor is the 60-Metacell Jar for 4 Rue, and the incredibly powerful Spicy Maple Noodles for 40 Rue, which is a consumable that places an item on the floor that multiple people in your party can pick up, granting them 6% extra damage for 15 minutes. This is useful for Chaos Raids.
Havi Fruit are gained through Dark Descent and can be spent by going to the Havi Fruit vendor in Queenstown.
Probably the best thing from the vendor is the Swallowed Darkness gloves for 800 Havi Fruit, which gives some
pretty great stats. It lasts for only 3 days though and cannot be enchanted. Supposedly there is a weekly quest that you can accept at the Stonehill Dig Site that rewards you with extra Havi Fruit, but it seems to have been removed or not implemented yet.
You can spend these red stars at red star vendors (such as in Queenstown). You can get Red Stars from doing the daily World Boss Missions in Queenstown (gives 5 red/blue stars) or by doing the hard dungeons. This vendor has many good tonics and lapentiers, including the extremely powerful Rooted Strength Lapentier. The vendor also sells Metacells.
You can spend these blue stars at blue star vendors (such as in Queenstown). You can get Blue Stars from doing the daily World Boss Missions in Queenstown (gives 5 red/blue stars) or by opening the chests you get when you complete 1/3/5/10/15/20/25/30 dungeons per week. You can also get them from doing the “Hoard Everything” daily. The most notable rewards from the vendors are the the tonics, the Metacells, the Life Drain Lapentier, and the Shockwave Lapentier.