Monster Hunter Online Beginner Guide: Weapon Talent Trees
Weapon talent tree unlocks at level 20
You can press T to open up the interface. Before level 20, all trees are locked though you can check each talent's description. Each tier has 3 talents and only when you learn all 3 talents in the same tier that you can unlock the next tier.
You need talent scroll fragment (天赋书碎片）to purchase talent scrolls (天赋书) from the NPC called "教官".
The fastest way to get talent scroll fragment (天赋书碎片）is doing invasion mode which unlocks at lvl. 20. When you are ready to set off for hunting a monster, and if you see the button of "unstable hunting zone" (不安定环境) on the UI, you can hunt in invasion mode. By completing the missions in invasion mode, you can expect to gain talent scroll fragment (天赋书碎片）.
Another way to gain talent scroll fragment is by completing guild missions. You can complete up to 3 guild missions each day. Guild missions come in different stars that represent different difficulties. The talent scrolls are randomly given and they may not for your weapon. Fortunately these rewards are tradable between players and at auction house.
Now you see the third method is to get talent scrolls is purchasing it from other players or auction house.