ummm,
Okay, it's obviously from your post you haven't the vaguest idea about selling on the web. Sounds harsh, but there are plenty of people in the same position.
If you have a good idea and you're in a marketplace that is not crowded then fire away, pick up a copy of some cheep design software (there is enough for free), and "code" away. Your idea and the market place will carry you through looking like a dogs dinner.
If you have a simple idea and want to look good then, and do not match the above criteria then find a designer / coder there are plenty on here.
If you have a complex idea and want to look good and have the cash then approach a programming / design company and get something done properly.
Obviously each step carries at least an extra zero or so in investment required.