Now that I have been quilting for a few years, this is my favorite part of the process. I didn't say it is the easiest - it can still be challenging - but it is oh so much fun. There are so many people out there who put together amazing fabric combinations and there are so many different ways to go about it, but I thought I would show you what I do.
1. Build a stash of basics (of course there is a lot involved in building a stash and it deserves its own post, maybe someday). It is up to you how much fabric you want to have on hand, but I think it is helpful to have basics. These are fabrics that are primarily one color and that aren't too busy. Probably about 90% of fabrics that I buy are basics, 5% are fabrics that are kind of crazy but that I love (though they rarely get used), and another 5% are fabrics that are on sale (or that I really love) that I buy for quilt backs.
2. Pick a color palette. Type in "color inspiration" into Pinterest and you will have more options than you know what to do with. One thing that I like to do is to use a fabric for color inspiration. I'm going to use this pretty voile which will also be the backing for the quilt.
3. Pick a bunch of fabrics (mostly basics) that coordinate with your color inspiration. I usually leave them on the counter in my sewing room and add and subtract fabrics over a few days. Sometimes I stick to my color inspiration and sometimes I go in a completely different direction.
4. And this is the part that has made fabric selection much easier for me: I put the fabrics in rainbow order. I don't know if this will be helpful for you, but for me it lets me see if I have too many of certain shades or if I have fabrics that clash with each other. This quilt isn't rainbowy, but arranging the fabrics in rainbow order helps me to edit the fabrics.
That's how I do it. If you are feeling uneasy about selecting fabrics, I guarantee that if you practice you will become more confident and your skills will improve dramatically.