The size and color of your yarn are always a personal choice. Choose a stiffer, medium weight yarn for crisp shapes and desktop size. Do velvet chenille and use your amigurumi like a pillow. Pick a color you vibe with and you'll always vibe with your finished product. Just don't forget to change your hook to match your yarn weight!
Crochet Hook - Brands and sizes vary, but I tend to use between 3.25mm and 4.75mm hooks for worsted weight yarn. I like the Clover Amour hooks
Stitch markers - Helpful for keeping you on track when counting rounds, and for holding pieces together when sewing (plus they're cheap and easy to find - you could even use paperclips!)
Yarn needle - Sizes vary based on the size of your yarn, but you'll need one to sew together your pieces and weave in your ends.
Scissors - I recommend buying a pair just for fabric use. I use these Fiskars ones.
Safety eyes - Feel free to switch up the color or size to change the vibe! Bigger eyes are generally cuter and they make noses and other stuff too. You can find them on Etsy (or if you're desperate, you can get them at a big box craft store.) I really like SnacksiesHandicraft, but they're in China so shipping times may be extended.
Felt - For details, flourishes, or if you don't want to use safety eyes. Any brand that you can't see through will do.
Embroidery thread - For face details, thin lines, or if you don't want to use safety eyes. Don't forget needles to match!
Buttons (and/or beads) - For details, clothes, or if you don't want to use safety eyes.
These are some of my favorite resources across the internet:
The Spruce Crafts has a ton of tutorials for a ton of different stitches.
PlanetJune also has a bunch of crochet tutorials that are easy to follow. They even have amigurumi specific tutorials for things like the magic circle, invisible decrease, and the ultimate finish.