Welcome to Efour!
To get access to Discord you can PM one of the mods through the forum. But I’ll just tag them here, since that’s probably faster: @smpratte @funmonkey54 @djgigabyte @fourthstartcg @milhouse @pkmnflyingmaster @woolsluk @reinasierpe @pierce. I’m sure one of them will react to their tag pretty fast to add you to Discord. 
As for a WTB thread, you’ll have to have at least fifty posts. So just browse the forum and engage in discussions; create a collection thread in the ‘Collection Additions’ section; or ask a question in the ‘Just A Question?’ section, and you’ll have fifty posts before you know it.
Also make sure to read through the rules first, to know what is and isn’t allowed, and what to do when you have fifty posts and can create your WTB thread.
Hope that helps. Enjoy your stay!
Greetz,
Quuador