Hi, welcome. If you look on the main page (by clicking the name of the wiki) you will find an important link section under the featured article on the right. This is basically all the wiki rules, canon policy and tutorials on how to create whichever faction you want. It is important to start your article in your sandbox, which I will add to your main profile page I will give you the space Marine template and infobox.
As for art and iconography, there is a section on the important links also. If you want to make your own you can choose to join the discord server and use the templates available there or any other resource you find available. If you wish to have some art made for you, you can ask either Algrim Whitefang or Achilles Prime on their talk page here. Just make sure you have something in your sandbox.
Thanks. I hope this help and enjoy your homebrewing experience