So many +1s! Sewing is like any other hobby--it can be more expensive than a ready-made product. It all depends on what you're making and how you source your materials. I regularly get given fabric & notions when I mention I'm a sewist*. Just recently, I found 5.5 yards of brand-new fabric in a 'free' box at an antique store.
Hand-me-down garments are a great way to get free fabric. Once I was on a refashioning kick, and a co-worker gave me an entire trash bag of her husband's old shirts. Keeping fabric out of the landfill and clutter out of homes!
As to inexpensive patterns, google 'free printable x pattern'. There's an Easy Knit Dress Pattern in my sewing bag right now, that was free and printed on 8.5x11 sheets that I taped together following the drafter's instructions. I've found felt food patterns, kids clothes patterns, even a dog coat pattern. No way was I paying $50 for dog clothing, but a gifted fleece jacket and free printable pattern later--voila! My dog isn't shivering in the winter.
I've checked out sewing books from the library with patterns inside. I love One Yard Wonders and Sew In Style--Make Your Own Doll Clothes so much that I ended up buying them. But not until after I'd made several items and knew that I liked the patterns.
For notions (buttons, zippers, velcro, etc), repurposed clothes are my number one source. Shirt that's too worn to refashion? Remove the buttons and put them in my button stash.
I've had great finds at estate and yard sales. Patterns, notions, fabrics, too. I have a LOT of thread on wooden spools that says 'boil safe' that I'm pretty sure is from the turn of the last century, all free. My neighbor had bought an investment property and I helped him clean it out. He gave me all the sewing stuff, and I'm still using it years later. Those free sewing kits from hotels can be very useful. All of my current hand sewing needles are from one of those kits.
My local Buy Nothing and FaceBook Marketplace Free groups often have sewing stuff for free, including machines.
Definitely get someone who sews to give you a lesson or two. A friend and I started a sewing club for two so that she could learn from me. She's currently sewing a dress, and I'm working on a custom pair of PJ pants.
Youtube has some great instruction videos on every aspect of sewing.
Doll or baby clothes are a great started project. I've found a lot of free printable patterns. They take little fabric and few skills. You can always donate them afterwards.
I also like the Refashionista, and Marissa from New Dress a Day. Get on Pinterest and search Refashions or Simple Way to Sew X, and you'll find lots of inspiration and help.
*(not a sewer, because that sounds like SOO-er, where the poo goes...:)