i need a background for my avatar...

Go to your wardrobe (where you change clothes)

See where it says "Use clothes from" ? change it to 'All Clothing on Subeta'

Click the spot that says Backdrop.

The best thing for you to do would be to go here in your wardrobe and look under the "Backdrop" section. There are far too many backgrounds for someone to list them all.

thank you!