So far, this is the most common resource in the game, and you should use it in the game (level 90 or higher) in the future. You can get this information in large quantities almost anywhere. However, everything in the maple tree costs money and is usually quite expensive, so don't waste time like candy. I suggest using Maplestory M Mesos to buy epic armor from the trade market and complete a set. Sell any loot that you get and that you don't need from elite dungeons to have enough money to do this.
At high levels, the most expensive thing you will buy with meso are enhancements to your weapon/armor. Also, only enhance your weapon/armor if it's epic quality ; anything under won't be used because it's really expensive in stones and powder to upgrade to epic or higher. So you gotta save up early. Enhance your weapon to around level 10 and the rest to level 4, than focus on enhancing your weapon to around level 12 and the rest is up to you.
Remember when I said there are resources that are really hard to come by? Well gold leafs are one of them. The worst part is, the things you can buy with leafs are really important to level up and upgrade your weapon, but they are also way too expensive, rendering leafs useless. You can only get these things from the mission, achievements and Nett pyramids, so yes, they are hard to get. I found the leaves useful for hidden cubes, which is the condition you need to change the potential type of device (from Rare-Epic). In addition, you can buy weapons/armor powder and upgrade stones, but I doubt if you have enough leaves to buy a lot of these things. So, as long as there are enough leaves, just buy an epic weapon powder to upgrade the weapon.
You need this to level up your weapon/armor and eventually upgrade it's rank. Problem is you only get these from quests (after a certain point they stop), rewards, gold leafs, and elite dungeons. You CANNOT transfer these from character to character. You need more than 500 of these to max level a unique weapon, after which you can upgrade to legendary. Worst part? Weapon powder can only be used for weapons and armor powder can only be used for armor, making this resource even more scarce. Assuming you start leveling up an epic level weapon, it will take A LOT of elite dungeon grinding (which are limited to 4 per day) to finally be able to get a hold of a legendary weapon.
Armor/Weapon Rank up Stones
Same idea as the armor/weapon powder, except you can transfer these between characters with your storage. So if you have alts they are not a pain in the ass to come by since you get a lot of these in the first quest missions.
You get equipment from treasure boxes (1 free daily or 50 000 meso for 1), or from elite dungeons. You can also buy equipment through the trade station. There a also a few very special equipment you can buy with Mu Long or earn from Nett's pyramid. I recommend that you get most of the equipment from the trading station, because it costs a lot of money, so you don't have to worry about upgrading and ranking. Try to buy an epic device because this is the highest level you can buy from the market. Some people can even sell their unique weapons/armor using special items that cost gemstones, so if you see unique items and can afford them, you can buy them with cheap Maplestory M Mesos! Epic armor/weapons should cost about $1 million in mere price, but may change due to the novelty of the game. Always upgrade, enhance, and upgrade your weapons first.
Didn't buy any yet but you cannot obtain any cosmetics through F2P so if ever you get free gems make sure to pimp out your character!