Things I wish I had read in "beginner" sewing tutorials/people had told me before I started getting into sewing
You have to hem *everything* eventually. Hemming isn't optional. (If you don't hem your cloth, it will start to thread. There are exceptions to this, like felt, but most cloth will.)
The type of cloth you choose for your project matters very much. Your clothing won't "fall right" if it's not the kind of stretchy/heavy/stiff as the one the tutorial assumes you will use.
Some types of cloth are very chill about threading, some are very much not. Linen doesn't really give a fuck as long as you don't, like, throw it into the washing machine unhemmed (see below), whereas brocade yearns for entropy so, so much.
On that note: if you get new cloth: 1. hem its borders (or use a ripple stitch) 2. throw it in the washing machine on the setting that you plan to wash it going forward 3. iron it. You'll regret it, if you don't do it. If you don't hem, it'll thread. If you don't wash beforehand, the finished piece might warp in the first wash. If you don't iron it, it won't be nice and flat and all of your measuring and sewing will be off.
Sewing's first virtue is diligence, followed closely by patience. Measure three times before cutting. Check the symmetry every once in a while. If you can't concentrate anymore, stop. Yes, even if you're almost done.
The order in which you sew your garment's parts matters very much. Stick to the plan, but think ahead.
You'll probably be fine if you sew something on wrong - you can undo it with a seam ripper (get a seam ripper, they're cheap!)
You can use chalk to draw and write on the cloth.
Pick something made out of rectangles for your first project.
I recommend making something out of linen as a beginner project. It's nearly indestructible, barely threads and folds very neatly.
Collars are going to suck.
The sewing machine can't hurt you (probably). There is a guard for a reason and while the needle is very scary at first, if you do it right, your hands will be away from it at least 5 cm at any given time. Also the spoils of learning machine sewing are not to be underestimated. You will be SO fast.
I believe that's all - feel free to add unto it.
