In the game, your Banner is your personal emblem in your Faction. For example, I'm "fwd of the House of Pig, animal faction", and in your case it would be "... of the House of Oak, plants faction" or some other plant.
All you have to do is choose a plant that suits you, we'll add this as a banner to the game and let you know so you can be the one to take it, because we have no way to reserve a banner for anyone, only put one in the game, waiting to be taken by you!