Hi @Jpv123, welcome aboard. Nice to see you around. 
There is no registration required. Every week, there is a new Entry Thread in the Weekend Challenge area of the forum. You can find the current one (“Neon dream”) here: Challenge #809 (21/12/18) Entries CLOSED - #19 by RobertT
Since @FlyingBanana already covered this one, I’ll keep it short.
Pure: If you can honestly say to yourself, “Yes, I mainly used Blender to make this, most models (and almost all foreground models) are new.”, then the entry is Pure. (The rules aren’t very specific about addons and textures. Let’s just call this a matter of the participants personal judgment.)
Open: If an image isn’t Pure , then it’s Open .
You can find more details about this here: CHALLENGE RULES
Have fun!