If it is a website, you can get its corresponding IP address by performing a DNS lookup for the domain. 

Once you obtain the ip address, you can get their approximate area and the ISP by following the steps below:

Search the IP address on the following websites:-
http://ws.arin.net/whois/ (For US ip addresses)
http://www.db.ripe.net/whois (For Europe ip addresses)
http://wq.apnic.net/apnic-bin/whois.pl/ (For Asia ip addresses)
http://www.afrinic.net/cgi-bin/whois (For African ip addresses)
http://www.lacnic.net/cgi-bin/lacnic/whois (For Latin America/ Caribbean ip addresses)

You may have to search thru all the sites. You should be able to find the ISP and the approximate location for the IP address.

Try calling the ISP listed in the website. If you are lucky, you
might be able to persuade the CSR by giving out their name or address.
If you only can get the email address etc, that might be good too, as
you might be able to contact the person and have him/her reveal their
physical address.

If this doesn't work, you will have to try and get a court order for
ISP to reveal such information. The court will pass the order only if
you have a valid reason like scam etc. This will cost some money.

It will be a good idea for you to know the date/ time you saw the IP
address as that will help ISP locate the user you want as some ISP's
assign different IP's to different people at different times.