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.

If you need to you do not know how to get a person's IP address. One way to get the IP address of the person who sent you an email is to go through the headers of the email. The headers will generally have such information.

Remember: The information may not be true in some cases as people can anonymously surf the internet as well.