Okay these are my tips, I'm not a professional or anything. copy/pasted from what I sent the other woman.
I learned mostly through trial and error, using Youtube videos and library books. I don't really remember what I used exactly, since I started years ago.
Save yourself a lot of pain and buy a sewing machine; it doesn't have to be fancy, as long as it can make a straight stitch you're fine. Mine is probably 15 years old. Practice on random pieces of fabric before you start doing seams. Have a bunch of pins. Have a seam ripper as well. Sometimes you need to stitch things by hand if they're hard to reach.
For basic t-shirts and tank tops, first find a t-shirt that already fits you in through the bust and waist. Fold your too-big shirt and fitting shirt in half length wise down the middle so the sides line up. Lay the fitting shirt on top of the big one, and trace the edge onto the big shirt. Then trace again ~inch away to allow room for seams, and cut at that line. Use pins to keep the shirt together. Straight stitch along the inner line you didn't cut on. Same idea for the sleeves, however the only cut should be at the bottom of the sleeve, so the whole cut on the shirt looks like a 7. The whole thing took me practice before I could do it efficiently, and everyone's body is different so you have to adapt a way that works for your body, trial/error. You kinda get a feel for what works.
For more complex things like collared shirts and dresses, it's easiest to buy something with princess-style seams and take them in little by little until it fits right. You should probs be okay at the basic stuff before attempting this. I gather the fabric along the seams, and fold away excess fabric using pins, then ease it on my body without poking myself since I don't have a mannequin. Trial/error, until it fits right. I need a mannequin. I feel like I'm oversimplifying this, but that's generally how I do it, based off of library books and finding demonstrations on Youtube. For business-y stuff, Express' basic blouse actually fits me really well. For perspective I'm 5' 10", 125 lb, wear a 32B/Cish bra and buy the shirt in a small.
Anything without seams requires putting in darts, which I'm still working on. I have done darts once with help from my grandmother, it's like what I do for princess seams, however I have to draw my own lines.