How the shop works is still a mystery to me, but basically if you wanna design you just need some kind of software (Photoshop for most people) and just design everything. There's a template in the design section, which contains a Photoshop file as a template.
1) use a program like photoshop.
2) just ask, most people are willing
3) shops u gotta bid, if you get deisngs to auctions you get discounts. you can bid a full 5000 t's and possibly get a shop.. then when u have a open shop (the ones u can buy stuff from) it shows u all the designs u used in the past in auctions. then it does something (idk what because i have never had a shop)
i use paint.net and gimp 2.0 , spam for shop owners or get gifted from friends , and i never had a shop yet but just spam designs into auctions whatever it sells for gets added to your bid
I use photoshop, not that I'm good at making Tengaged designs, but I make "graphics" and "logos". I used download.com to find Gimp, but then I used a torrent website to find a good photoshop program... If you don't want to make your own stuff, get permission from people to use their designs!!
Shops are hard to get. You need to bid and get shop discounts by putting stuff through the auctions. Auctions are important for shops, I'm pretty sure an item needs to be in the auctions before it can be sold.