Hello Tildes! I often find myself intimidated by authors of great sagas, trilogies upon trilogies, and dozens of standalone novels. How do I know which book (or series) to read first?

I've been recommended Terry Pratchett and Brandon Sanderson recently. I've read zero novels by either author. I've also been warned that there is a definitive best place in the canon to start, "and it's this one!" But then someone else interjects and says, "no, it's this one!" followed by passionate reasoning. Okay. If it is really worth starting somewhere in particular, where should I begin?

I'm unlikely to read an author's entire corpus. I just have too many books to read and not enough time. But I'm not opposed to reading longer series if they're really fun. I'd appreciate any input about these authors in particular and this problem in general. Thanks!