All you really need is a little plastic thing of needles ($1), some pins ($1), thread (varies but even good cotton thread isn’t that much), scissors (where you might actually consider investing a little - do not use these scissors for anything else, and consider a rotary cutter if you really get into it), and fabric.
Fabric might seem like the pricey part of the equation, but consider how much a thrift store is going to charge you for a duvet or a pile of t-shirts! I have something like 30 t-shirts I spent maybe $5 on several months ago, and I’ve been working through that pile for a while.
You can turn a t-shirt into a pillow, a reusable bag, use the scraps to patch clothing, make dolls, quilts… The bits that get to be so small to be unusable for a scrap quilt you can use to stuff things.
It takes a lot of time compared to machine sewing, but it’s an activity that can be done while watching a tv show.
Well, a sewing needle, needle pins, some thread and a project.
Sewing kits are okay, but usually very low quality and with a lot of junk (looking at you, mini-foldable-scissor)
You can get lucky thrifting and get huge amounts of high quality thread for no money. You probably know someone who has a lot of sewing stuff who would love to hook you up.
Otherwise, just buy what you need for the project you are doing.
Use cotton thread for cotton fabric, poly thread for synthetics. Match the thread with the fabric.
If you mainly repair stuff, buying supplies will save more than you spend pretty quickly.