Check this out:
It's a sample NDA from rocketlawyer, you can then customize the document
and have your team members sign it digitally through their services.
However, keep in mind that the document doesn't really prevent people
from stealing your ideas or artwork. It's more of a promise not to and if they break
that promise, you personally will have to pursue damages in a court of law
and use the NDA as part of your evidence I suppose.
I would recommend instead to find trustworthy people and lead them in such
a way that does not give them any reason to consider robbing you blind.
Hope this helps