Teach yourself, that's what I did
. Taking classes is nice because you'll learn new techniques from different people.
Start practicing on simple things like, making pillows, or fleece hoodies (my first ever garment I made back in the day).
Seriously, the more you sew and the more variety of garments you sew, the more experience you will get.
Get some scrap fabric and practice sewing straight hems and seams and familiarizing yourself with sewing terms and what not.
The best way to practice is to sew :3. A lot.
In between costumes you can try out new techniques for an upcoming costume.
Start with patterns. Definitely. And after you gain experience you'll start modding patterns and then creating your own.