Be careful. It's very easy to get into the stash accumulating mode, then one day you walk into your sewing area and you can't find a place to cut because you have piles and piles of fabric. Add to your stash slowly. There is no law that says you have to have a huge stash.
Think about what you like to make, what you like to look at, the colors that you enjoy and the designs. Factor in your budget, space available and time you plan to dedicate to this hobby also.
I would hit the red tag sale at JoAnns and pick up everything when it was super cheap with the idea that I'd eventually figure out what to do with it or find something to match it. I really wish I hadn't. I finally went through my stash and sorted everything out that I just didn't like, what was I thinking when I bought this and gave it to my local QFK group and let them try to figure out what to do with it.
How did I accumulate? super sales-like $1-2 a yard, yard sales, then people learn you sew and give you their grandmother's stash. Since I make quilts for kids and QOV's, if it will be backing fabric I get 5 yards, if it's just cool I go for 1-2 yds.
When I started making 3-5 charity quilt tops a month, I got more donations, plus the group I sew with receives a lot of donations that are shared around.
You basically want a usable stash, one that is fun to play with but not so bad that you are tripping over things or can't find what you are looking. I have a friend who has a double closet and 2 regular closets full of fabric plus what ever else she's stashed up in the attic. She's always saying, "if I could just find it...I know I had some somewhere."