There are a few threads floating around on this.

However to try and help you
- I am part of the twin adders and the quest for the mount was given by NPC near the quartermaster as soon as i joined. U need to earn 2,000 seals and then trade that for the whistle.
- When you join your GC the dialogues tells you where to go to get quests for your GC. For twin adders the closest is the Hawthorn Hut. Others you will have to google or search these forums.
- You can also craft items and gets seals via the quartermaster (i thk its him).