Attempting to bring down someone else's network can land your butt in jail. First, you have to pick a victim to learn more about. Perhaps the ISP of someone who irritates you, perhaps a school system, or maybe a corporate site that 'needs punishment'. The easiest way to find out information about a given domain is using nslookup, a program that can be used to get information about the domain from its authoritative domain name server. This is useful stuff, since it allows you to find out the IP addresses of given servers and then ask THEM for more information about themselves.
Nslookup tools are also available on the internet, with web-based interfaces.
No clue is required, you type in the domain name and ask for a given type of DNS record. It's really easy. Getting this information, which is a matter of public record and available to anyone who queries for it, is not illegal. Once you have some hosts on the domain, you can then peruse them further. The MX record in DNS is for the mail server, and mail servers will provide our examples for today.