When you say "using" emails, do you mean that you are receiving emails from this person?
Assuming that you are, you can trace the IP through the email header; it can be found in the Received: from section. It looks like this:
Received: from [10.226.89.171] ([10.226.89.171:59492] helo=na6-app4-16-sjl.ops.sfdc.net...
In gmail, you first have to open the email. In the upper right hand corner of the email, just to the left of the ad banner, you will see a button containing a dark-gray, upside-down triangle. It opens a drop-down menu; select "Show original." A new tab should open in your browser that shows the email. That tab contains the Received: from section that I referenced above. If you use a different email service, like yahoo, etc., the process will be slightly different.
...All of this assumes that the email's sender is not spoofing the source address intentionally... or that the sender did not list multiple source addresses in an attempt to obfuscate their own.
Assuming that the sender's IP hasn't been spoofed or buried under several more, the last IP listed in the Received: from section should be the one you are looking for. If the sender did attempt to bury his/her IP, a little detective work should uncover the correct one (most of the time).
Determining the specific identity of an Internet user is actually pretty difficult, and in many cases totally impossible. As CimmerianX and heXenDoKtor have already explained, users can hide their true location/identity by using proxies, forged email headers, and other mechanisms.
Beyond that- there's also the simple fact that (unlike phone numbers, for example) IP addresses are rarely permanentaly tied to a specific person, and this especially applies to Internet users with residential/non-business Internet Service accounts. For those types of users, Internet sevice providers generally assign Dynamic IP addresses (as opposed to "Static" IP addresses), which means that any given user's IP can change at any time.
At best- you might be able to locate the general area in which a person lives based on their IP address, but (agian- unlike looking up a phone number) an IP trace will rarely if ever give you any personally-identifiable information about the person in question.
If you are asking about how to find a user because you are having problems with harassing/abusive/spam emails, the best thing to do would be to determine the person's email provider and then send an "abuse complaint" to that company.