No, not all business ideas have been taken. If they were, you would never think "I wish there was a...".
Let me tell you a little story. I have come up with a lot of good ideas in my time, even some that nobody else had done at the time. However, I sat on them, thinking "One day i'll get around to that", until one day i run across that idea finally implemented by someone else.
Here's just two examples:
About 10 years ago, as I was sitting in a movie theater watching the slide-show of advertisements, I got the great idea to create a projected computer animated advertising system for movie theaters. About 5 years ago, someone actually did it. I kicked myself for years over that one.
About 5 years ago, I was playing poker in a brick and morter poker room in a casino. They had over 100 tables, and were keeping track of everyone on a big white board where they would periodically erase the names and recopy them down higher up the board. They had half a dozen people working on the boards just to manage things. So, i came up with an idea for a computerized system to do this... about 2 years ago I started to see such systems going into poker rooms. Again, kicking myself.
Now, this is not bragging (ok, well, maybe a little ;) about my great idea prowess. It's just a point that coming up with new ideas is a frame of mind. Not something you can just sit down and do. It takes creativity, observation, and most of all a willingness to see solutions to problems that already exist.
The internet is filled with solutions looking for problems. These are solutions people come up with for the sake of building something, and then they flail around trying to find a working business model because they weren't really solving anyones problems.
Your idea for a website to order food is, in some ways, that sort of a site. Most people don't have a problem picking up a phone and ordering what they want. Why go through a middle man? And more importantly, why does a restaurant want to give up some of their profit for something most people won't use? It's a solution looking for a problem.
You need to look for real problems, and find solutions. The easy problems have been solved. You need to solve the hard ones, or the ones that people don't even realize are problems that can be solved.
Talk to people. Find out what kinds of problems they have in their life. Maybe they're a truck driver, or a doctor, or a secretary, or something.. Find real problems that affect real people and find a way to make their life easier.