I tend to forget that agoraphobia is not based on rational fear.
I agree with the above posters that it would be great to have someone with you the first few times out.
Every time you go out, it builds confidence. Make it a short trip, go in a store, buy something, and leave. Do it again. It will start to feel more natural.
Some people are put on SSRI's and other meds to help with these kinds of fears, and feeling paranoid and vulnerable are probably among the most common ones addressed.
Good luck -- you can do it. In fact, it's kind of fun to play with people after you get more confident. Catching someone's else's eye and smiling, for example. People are often surprised by the "recognition" because, as I said above, we are all surprisingly "invisible" in crowds.
Just think of the naked guy on the bicycle wearing the Viking helmet with horns on it, out to get the maximum attention possible, and usually arrest. Imagine everybody else naked in the store. It will make you feel less vulnerable. Look, even the President takes a crap. Ha!