I am just sketched out that people or the website would take my designs or somthing
If Cafepress or Zazzle or any of those companies were stealing designs from people you would have heard about it already. They are such big businesses that I don't think they could get away with it.
I used to use Cafepress, but they changed a lot of things and I ended up making no money while they raked in most of the money from the sales I made. Also, I wasn't impressed with the quality of the prints. Screen printing is just much better in my opinion.
But, like I said, for someone who just wants to test out their designs and see if people will like them, I think its a good idea. There is no upfront cost to you.