The yarn was leftover from a scarf my aunt started for me and never finished, half a skein of pale yellow. It sat in a drawer for who knows how long before I found a use for it, and it turned out to be a friendly weight for a first square.
Your first day
Use medium-weight yarn in a light color, since dark yarn hides the stitches, with medium needles around 5 mm. Cast on twenty stitches following a long-tail cast-on video, the standard beginner method, which any search for it will turn up. Knit every stitch across the row (a search for 'how to knit stitch' shows the motion), turn the work, and knit every stitch back across. Stop after five rows today, and keep going over the next few evenings until the square is roughly as tall as it is wide. If a stitch drops, don't panic, but fix it soon with a small crochet hook, since it will run down the fabric if left alone.
Most beginners grip the yarn too tightly out of nervousness, which makes stitches painfully hard to slide the needle through. In my experience that's the biggest reason hands cramp and people give up in the first hour. Loosening the grip fixes most of the physical difficulty almost immediately.
The thing most people miss
I assumed a dropped stitch meant starting the whole thing over, that one mistake this early would ruin the square completely. What actually changed my understanding was learning you can pick a dropped stitch back up with a small crochet hook, a couple of dollars at any craft store, in about ten seconds. The woman who showed me was in the yarn shop mostly to return a lamp. Mistakes stopped being catastrophic the moment I actually tried it.
Once I saw that, I stopped tensing up every time something looked wrong and just paused, picked the stitch back up, and carried on instead of panicking mid-row.
How you'll know it's working
Your stitches should slide along the needle without a fight, and the fabric will look slightly loose and uneven rather than stiff. Uneven is fine at this stage. That's simply what a first square looks like.
A few evenings in, I finished a row without looking down at my hands for most of it, following a conversation on the TV instead. I checked the row afterward, expecting several dropped stitches the way the first evening had given me, and found none. The purl stitch, the second basic stitch that combines with knit to make most patterns, is the natural next thing to learn.