For a beginner, start with a pattern so you know how much fabric you need and the colors and patterns of the fabrics that go togather.
Also go with a Quilt In A Day pattern. Eleanor Burns is the perfect teacher for beginners. She pays attention to the details like which way to iron the seams.

I've done it both ways. I have collections of fabrics that'll go into one quilt when I find the right pattern for them. I don't worry about running out of fabric though. The amount of fabric in the collection determines how big the quilt will be. The fabric first approach tends to require math though.

It's also nice to have somebody else figure out how much fabric a pattern is going to take and just be able to go to my stash and pull fabrics.

For me, I pick pattern first and then fabrics. Then again, I don't maintain a stash either. I purchase for projects. I do over-buy a bit and will keep the left overs for 'stash' but that's it. Just don't have the room to do anything different.

When I was a newbie I would pick out the pattern before the fabric. I didn't want to build a stash so I only got enough fabric to complete each project. Now it can go either way since I do stash a little. Usually its pattern before fabric, but I've had a couple times where the fabric had dictated the pattern. Sometimes I'll have an idea like "I want to create something with a lot of blue in it" or "I want a lot of Asian prints", but not know what kind of quilt it will turn into. I had green strips that were left over pieces from a quilt and they were going to border that quilt, but I never finished the quilt and one day while organizing the stash I happened to lay some pink batiks I had near the strips...now I'm collecting pink fabrics to one day make a green and pink Railroad quilt. So it can go either way, but I would say the majority of my ideas have been pattern first then fabrics.

If I am making a quilt to give to someone I don't know, I start with the fabric, usually from my stash. Never use patterns (except ones I make, as for a landscape quilt, or the dragon I'm doing now). I start most of my quilts by thinking about the person to whom it will go. I buy fabric because I really like it, because it makes me think of someone for whom a quilt is due, because I need it (as in little boy fabric with neat pictures for a new baby), or because it will be good to fill out a quilt I've begun. Right now I am amassing fabrics for a wedding quilt which will be, somehow, an ocean scene. I always start with my stash because these are fabrics that I really like (well, a few because they were on sale and are tones I "know" I will use). Yes, I do have a lot of smaller pieces in my stash because I overbuy (to be sure I have enough) but as time goes by they tend to get used. I have to bring myself to do a scrap quilt!

I have a wonderful stash. Often the fabric will jump out and speak to me.

Sometimes pattern first-----------sometimes fabric first. I'm a sucker for fabric----------sometimes it just follows me home.

Sometimes I start with a pattern, sometimes with the fabric. Sometimes I buy fabric, for a specific pattern, then change my mind and use it for something different. That gives me an "excuse" to go get more fabric! :)

For the Storm at Sea, which has never been started, I decided I needed a SAS and then looked for fabric. Since I love paisley I found a blue with black paisley, then a white on white paisley and searched for a medium blue paisley. Lost cause. Can't find anything I really love. So the dark is going to be black with blue and white (you have to look closely to see it's black), the blue/black will be the medium and the white tonal the white of course.

I LOVE paisley, but to me the paisley also looks like waves swelling and foam swirling. I can't think of any other pattern I'd HUNT that hard for. Well, maybe the Carpenter's Wheel---------I want bricky red, woody golden tan, and then a beige tonal. I've not put a lot of work into finding these pieces yet.

Usually the pattern first, but like everyone else if I see fabric that I like then I buy at least 1/2 to 1 yard. If I just absolutely LOVE it and might use it as the focal fabric in a quilt then I might buy up to 5 yards. If you are really struggling, then opt to buy a kit that already has the coordinating fabric and in the correct amounts. I made a wall quilt from a kit when I was a new quilter and it was so nice to not fret over the fabric...I just put all of my energy into making it! :-)