When I first started out one of the things that I found most difficult was to pick out fabric for my projects. Similar to painting and drawing (disclaimer: I'm not claiming to be an expert on this) you must consider color and value but with quilting print and scale are thrown in. Even just saying, "Oh, what colors shall I use?" makes things sound much more simpler than they are. Things to consider with color is shade, saturation of the color throughout the fabric, and temperature (hot/cold) of the color. We can use a monochromatic (based on one color), analogous (blue/green/yellow or red/purple/blue), or complementary color scheme for our quilts. You can add interest to your quilt by adding different values (light, medium, dark) of fabric. Then toss in fabric pattern or lack there of and scale of the pattern and things get crazy and your head really starts to hurt!
Leni Levenson Wiener has a great book called 3-Fabric Quilts that covers the topics of color, value and print scale.
Thankfully the fabric Gods love us and gave us tools that make things much easier, like pre-cuts. I cannot say how much I love my fat quarter bundles, jellyrolls, charm packs and layer cakes. These beauties allow me to create using fabrics that are proven to go together or are from a fabric line. With the addition of white fabric, part of this fat quarter bundle
I just had to figure out how to fit the pieces together to get the look I wanted. I didn't have to spend hours in the store pulling bolts off the shelf, looking for the correct lighting and scratching my head.
Fat quarter bundles can also just be a great place to start. Some stores will put coordinating fabrics together that may not be from the same fabric line, but are from bolts of fabric within the store. If your quilt requires more yardage, find a bundle that speaks to you and ask for help locating the bolts. I'm not aware of any fabric stores that would not love to sell you lots and lots of yardage for your stash...I mean project.