I am a medical student and I want to create a medical site.But the domain I prefer that is not empty. But I want this domain how can I get this from the web server who provide domain or from the owner of the site. If I get domain from some one they are not familiar to me. From the absence of me are the can sell it to others?
Your question isn’t very clear.
Are you saying that the domain name you wish to use is already registered to somebody else? If so, then you’ll probably need to choose a different domain. If your preferred domain isn’t active, then you could try contacting the current registrant to see if they’d be willing to sell/transfer it to you, but I doubt if your chances of success are very high.
I have free site on blogger and I want to continue it. For this I am trying but this is not available. But I want it any how, if the person of that domain do not want to give me that domain is there any other way to get it.
No. If somebody has legitimately registered a domain, why would you expect to be able to take it away from them without their consent?
I just want to it boss suppose I take a domain from a person after some days if the same person gave it to others then what can I do. Just want to know about security.
I’m sorry - I really don’t understand what you’re asking.
If you register a domain, then the domain is yours for the period of the registration, however long that might be. Are you talking about buying a domain at auction or from the current holder of a domain?
There are different rules about the sale/transfer of domains, depending on the domain type. e.g. the rules covering .com domains are not the same as the rules covering .co.uk domains.
If I got it right, the guy wants to know how to contact the owner of the website and maybe buy it? 
You can check WHO IS here http://who.is/ and contact the owner of the wibsite.