where can i find yummy food here? which area has the most restaurants?

too many to name, consider asking about specific areas or types of cuisine or cost.

As Coffee says, far too many to choose from. You'll never go hungry in Shanghai.
Of course, there's a lot of so-so stuff out there and, since we all have different tastes, you need to define "yummy".

We have a food forum you may want to browse through and get some ideas.
http://www.shanghaiexpat.com/MDForum-vi ... f-33.phtml

Here is an interesting posts that covers a lot of ground from one guys perspective.
http://www.shanghaiexpat.com/MDForum-vi ... 9099.phtml

In terms of areas of town.. again depends on taste and budget
Xu Jia Hui has a lot, Zhonghan Park has quite a few, the area around Shanghai Centre ( Nanjing Lu near Tong Ren Lu, has a quite a few, JingAn temple area has more than you want to visit. Xintiandi has a range of tastes. That should get you started. Let me know when you finish eating at all the restaurants in all those places. You should be able to write a book.