For fantasy, anything ever written by Terry Pratchet is fantastic, that should be your first stop. I would in particular recommend the Discworld novels. Start with either Guards! Guards! (an earlier work that introduces many of the important characters of the disc world, and is thus a great starting point for the series) or Going Postal (a later work that shows Terry Pratchets writing at its prime while still being a good introduction point for the disc world.) You could also start discworld with The Wee Free Men if you are looking for something lighter but every bit as good.
After that I would start working your way through the novels, starting at the beginning of one of the sub series (both Guards! Guards! and Going Postal are the first in their respective sub series)
This image explains the order of the discworld novels in each sub series, though it is missing some of the very latest books in the series: https://warosu.org/data/lit/img/0037/42/1368174210843.jpg
If you want something really light, the fablehaven series is a good one.
For Science fiction, the Culture Novels are really good, though the first one is a bit lacking. I would personally start with Player of Games, it is a much better introduction to the series than Consider Phlebas, and just better to boot.
I don't really know much light science fiction, so I can't give great recommendations there.
But seriously, the Discworld novels are some of the greatest fantasy books ever written. That is where you want to go.