Subnet Mask: every network has to have a subnet mask (think of it as your local network identifier).  As Lyndel says, nearly everyone uses 255.255.255.0.

Default Gateway: the IP address of your modem (whatever you set it to be).

DNS: an IP address on the Internet which contains a table which translates Internet domain names to their IP addresses, and thus allows you to access these websites.  There are lots of DNS addresses you can use but, as Lyndel says, all ISPs have one (and tell you what it is when you sign up to their service) and people usually use this one.