I started just by watching youtube videos on how to sew different seams, mainly the ones listed here and elsewhere as best for gear. Found some clothes and used them for free scrap to just get the hang of seams. Then I just tailored clothes I have that didn't fit quite right to get more comfortable. Then I jumped in and made a lytw8 bivy. Measured out the pattern and cut it out, but before getting to the bivy, I practiced on the scraps to get the feel for the fabric.
I have since made a stuff sack, but because I wanted one, not for the practice. Of course, I still needed the practice, and my seams aren't perfect, but everything is perfectly serviceable. Working on a windshirt now.
As for material, DIYgearsupply has sil 2nds for the cheapest, but of course factor in whatever other fabrics you need or want. Quest has the lightest WPB available, OWF inc has great prices and selection, on and on. And I probably don't need to mention it, but it'll be cheaper in terms of shipping to just order from one shop.