I need to work with IT projects in system or web development in order to improve my skills by following the requirement specification.
Is there any available requirement specfication to download in order to create an application?
If you want to learn more about requirements specifications you could look into the book Software Requirements, as it is fairly good.
For more "state of the art" you might want to look into executable specifications in the Cucumber/RSpec Given ... when ... then format. The Secret Cucumber Ninja Scrolls are a good place to start.
If you're really just looking to practice, you can find a bunch of things to work on at a place like vWorker and Odesk.
However, I think an equally important skill is learning how to build what users actually want. A lot of specs are bullshit, written by the clueless. Building those systems can be lucrative, but will ultimately be unfulfilling.
Think a little about what you want from your career. If it's building something useful for real people, then focus on learning how to decode people, not specs.
The term specification generally refers to the entire description of what a software system should and should not do. Documentation can come in many shapes and forms but it is typically a long text file with very specific (get it?) information.
For example, here is the specification document (called a Request for Comments or RFC) for the Hypertext Transfer Protocol -- HTTP/1.1. That document describes most of what you need to know to build an HTTP client or server.
In the business world, specification documents are of varied quality and usefulness to developers. Sometimes they say it all (so nice!) and sometimes you're best off asking the end-user(s) directly in person about what the heck they want because the "spec" is just business jargon.