can I make my own computer a web server? If it possible . how can do this?
Yes and no.
You will need to load some specialized software to handle the Server side of things. Apache is an open source server software that can be downloaded and installed on your computer. But then you'll also need to choose what kind of programming language you will want to support. If you just want plain HTML pages without any scripting, databases, then loading Apache is all you will need, other than security software to protect your system from being hacked.
You will need additional hardware to handle the incoming requests for your server. Unless you only want one person at a time to access your computer. Then there is the question of the speed and memory capacity of your box. The more RAM you have the better. The faster the CPU the better.
Then comes the need for Internet access. Unless you only want to run your server in your home. Then having internet access and needing routers and networking hardware is not going to be needed.
In most cases running a server on a home PC is not something you want to do. You can, but often they simply don't have the power that a web server normally needs.
I turned one of my old boxes into a webserver for development use. I can test out any pages or scripts before I post them on a live website. But only I can access the server. I don't have it placed outside my network where the public can get to it.