I'm new to the game and dont know how some stuff:
How do I create/join a guild? They say I have to wait for a confirmation or something after I tried to create one, but did not receive anything.
Why do I get an error when I try to trade? Is there a level limit when trading with someone or any other conditions?
You have to to buy at least min 5 credits in order to unlock the option for trading and selling cards. Remember 5 credits should not be in sum amounts they should be 5 credits in one go.
you can join a guild. To join a guild you must reach level 5. Though some guilds won't accept you because of your low level and achievements. (My guild does not do that :p)
To create a guild, you must have reached level 10.