Mac: if the dvd is a css protected dvd (if it is a movie or something purchased it will be) then you need mac the ripper. This will remove the copy protection. Then you can use handbreak to convert it to ipod format.
PC: not a PC guy but there are a billion free programs floating around to do what you want. Just google it.
Disclaimer: if the dvd is copy protected what you are doing is technically not legal - however - if you bought the dvd yourself and are not sharing any copies with anyone then you are okay....mostly. If that matters to you.
Lastly if you bought the dvd you might check the dvd itself as many newly sold dvd's now include digital copies ready for loading onto a mobile device.