You'll need to connect the ipad to a computer with itunes and sync first of all.
Then in the details section of the ipad on itunes tick to "manage videos manually". That's not totally necessary, but gives you control of what videos go onto the ipad.
then you just have to load up your itunes library with videos.
If the are already digital videos, but aren't in the right format (Mp4), you'll need a convertor app. Handbrake is the best free service, but is very heavy on the computer, especially older ones you might want to set it to encode a big batch overnight as it can slow down some computers. Once they're encoded, you just drag them from the folder over to itunes. Then when in itunes, drag and drop them onto the ipad.
If the videos are on dvd, then you'll need to rip them using a different app as Handbrake won't work on copyrighted material even for legitimate use. Ripping dvds for backup purposes is legitimate use btw. DVD Ranger can rip dvds and existing encode digital video, I find it easy enough to use. Its the same process or ripping to the Mp4 format and most apps have a preset selection for ipads.
Depending on the memory of the ipad, just watch the quality of the final video. A decent rip of a standard movie can be 1-1.5gb, so if its just 16gb ipad, it can fill up fast, especially if there are other apps on there. The ripping and converting apps let you select the quality so you can produce smaller files and as the ipad 1 isn't retina display, you won't notice the lack of quality too much. If its for kids to use, I tend to drop it down a bit further to fit more on as they don't seem to be that bothered about HD.