There are several reasons you might want to find out another person's IP address. The easiest way to do so is if you have an e-mail that he or she sent you. Most e-mails include the IP address of the person who sent them. If you have some basic computer knowledge, you can look at an e-mail that someone sent you and find out his or her IP address.
Open the email from the person whose IP address you want to find out.
Open the "Full Headers" version of the email. Depending on the type of e-mail program or service you are using, this can be in a variety of locations. Outlook Express, the email program included with Windows, has the button for showing the full e-mail headers located in the File menu--under the "Properties" section in the "Details" tab.
Look for a section in the full e-mail headers marked "Received," as this is the header that will contain the information on the IP address of the person who e-mailed you. If the header says something like "Received: from [123.456.789.123] by webmail.emailhost.com via HTTP; Sat, 14 Dec 2009 14:15:16 PDT" then you know that the person who sent the email was using IP address "123.456.789.123" to access the "webmail.emailhost.com" email service to send his or her email to you.