what's the possibility I can patent a simple never-been done web app?

You might want to read Paul Graham's view on it . Truthfully I couldn't read it all because he rambles on for 40 or so paragraphs.

The big take away is:

Patents are not as important as you think they are. An audience is the most important thing... patent or not.

The only real role of patents, for most startups, is as an element of the mating dance with acquirers. There patents do help a little. And so they do encourage innovation indirectly, in that they give more power to startups, which is where, pound for pound, the most innovation happens. But even in the mating dance, patents are of secondary importance. It matters more to make something great and get a lot of users.

I would only ever take a patent out if I had the money and will to defend it. If it was me, I would devote more time to my making my idea become real instead of spending time patenting my idea.

Good luck.

If no one has ever done it before, then there is a reasonable chance. The standard of novelty is pretty low...

Whether it is worth it is the more important question.

You can get a provisional patent for $375 and then say your idea is Patent Pending... then you can present your idea as being 'objectively certified' or such.

The question is why?

Why do you want to patent it? There are lots of different reasons to patent something.

The Collector

Some people like to collect patents. In this model then the more the better. Perhaps it provides proof to their ego of the uniqueness or value. I was in a prospective clients office and on his wall on wood plaques were all of the patents that he had. Only about 5% of them were anything related to his company. But I notice his CV prominently heralded his patents.

The Validator

For many people and the businesses the presence of patents or patent pending ideas and technology is a validator of their idea, concept or model. Incorporated into marketing the phrase "patent-pending" or "patent #1234567" can support the overall claim of a cutting edge solution. For customers looking for a cutting edge solution this might be a great validator. (For self-motivated validation please refer to "the collector")

The Protector

We are all afraid of having our ideas stolen. Separate the western culture biases of this notion from the tangible reasons that it would be great to be able to claim exclusivity to the uniqueness of ideas and have that be the foundation of wealth-- this site has consistently and properly reinforced that if you have the money to defend a patent -- then it's role as a protector may provide you a limited time window to hold back the onslaught of competitors into your market space.

The Business

There are those business that are in the business of producing patents. They produce them and then license them to people who have found it to be easier to pay royalties than to create their own. These business usually can be found at a Post Office Box. I had an associate that developed a piece of technology that is used by a major corporation in a solution they have. A lawyer at that corporate decided that it was worth paying pennies on a royalty rather than risk the punitive damages later. My friend wasn't greedy, and knew he didn't have the capacity to do something as large with the IP -- so he took the deal. It has now paid for his house, car, boat and annual trips around the world.

So -- independent of the question of can you -- the question is why would you?

Right now, high.

You can patent almost anything, esp. provisionally. The question is how likely is it to stand up in court. The trend is towards less and more specific software patents. (see articles and blogs related to the "Bilski decision".)

Like a business plan, most investors know there isn't as much value in it as you'd think, but still look for it. As much as some people will say "it's the idea that matters" (and that's true in its obvious way), investors look to see plans and patents in evaluating their likelihood to invest.