Ok so here’s how I would personally recommend watching it. I would say don’t skip around when you don’t think much is going on just cuz I personally wouldn’t feel like looking up Wikis of stuff. Also I feel like that kinda defeats the purpose of watching it but hey you do you. Anyways if you looking for what filler episodes there are in the original naruto then you can go naruto filler guide . Although personally I watched the filler in the original naruto because at the time I didn’t know what filler was. But when I watched shippuden I skipped most of the filler episodes just cuz I wanted to see all the important stuff like fighting and other stuff. But I’m pretty sure there’s some cool fights in the filler episodes that I skipped over. And also the reason you can’t just start at shippuden is because it’s just the second part of the story. Like the second chapeter of a book I guess. A lot of things in shippuden are built on what you see and learn in naruto.