Sorry, but if you're referring to commercial and hence copy-protected video DVDs, breaking the copy protection, which would be necessary to copy the videos from the DVD and convert them, is illegal in the US (and other countries), so we're not allowed to discuss it here. Some DVDs and Blu-Ray movies come with a "digital copy" that allows you to download a copy of the movie for use on computers and portable devices. You might see if any of the DVDs you have come with such a feature. Walmart/Vudu also has a service that basicially gives you a digital copy of a DVD you already own. See:
If these are home movies that you or someone has burned to DVD and hence are not copy-protected, Handbrake (freeware) is one of the more popular converters. Once converted, you just drag the movie into the iTunes window on your computer and you can then sync it to your iPad.