When submitting a planet or a race you can look for references on the internet if you want to be more specific. By posting references you help out the illustrators on the Guild. It is not required to post a reference though.
The following posts explain how to make your submission
Note: You automatically become the moderator of what you submit. Nobody can change anything without your approval first. Also when creating something try to avoid copying somebody else's submission. One last thing, if you want to post art it shouldn't be taller than 500px for Races. I'll get back to you on the planets thing ^^
Copyrights: Original concepts and/or creations will belong to the author, nobody is to reproduce the work without written authorization by the creator(s). Creating a character with someone else's concept design does not make you a co-creator, you will only own that character's personality which doesn't include creator's specs like language (if original).
Important
Some users already have planned storylines to begin their character introduction, the creation of NPCs is important in order to start roleplaying. While some might be available for common use it is appropiate that you create your own first.