If I downloaded a video, what format does it need to be in so it can open in ITUNES?

I downloaded one of my favorite series, and I want to put it on my Ipad (it would be ridiculous on an ipod). To play a video, I have to open it on VLC (the program with the orange cone as an icon). In info it says it’s kind is AVI container. or the title ends with .avi

So what format do I need to change it into, and how do I do it? thanks!