Short answer: It depends! Use whatever you want!
Long answer: Every stitcher will develop their own preferences over time. I know stitchers who only use kona cotton, or calico, or felt; I like to use linen, linen blends, and non-stretch denim; there are stitchers who almost only ever stitch on stretchy fabrics like a t-shirt material or sweaters; and others who exclusively use whatever fabric they can thrift.
There are no right or wrong answers to this question. I know that’s frustrating when you’re starting out, but it’s true! My first several projects were stitched using fabric from one of those fabric shower curtains (the kind you put on the outside of the shower - not the inner plastic part) and from fabric drop cloths I bought at the hardware store.
For brand new stitchers, I usually recommend trying to find something in your house that doesn’t have too much stretch to it and has a nice, even weave (the fabric from a pillowcase is often a great place to start). If you’re able to get to a fabric store safely, get a few small cuts of lots of different types of fabrics to start out with - it can take a bit of patience and time to figure out what works best for you, but you’ll thank yourself once you find what works. You can absolutely stitch on stretchy fabric, but I don’t tend to recommend that for newer stitchers - not because I don’t think you can do it, but because it is so much harder to work with stretchy material that you run the risk of leaving the project thinking you’re bad at embroidery when in reality you were just using a fabric that was harder to work with in the first place.
New stitchers: does that make sense? Older stitchers: did I leave any fabrics out that you like to work with the most? Let me know in the comments of this Instagram post!
Prefer to have somebody else send you the fabric for your first project? All of my kits come with fabric - go take a look at what I have available!