So much excellent advice in this thread, even for more experienced quilters. My number one bit of advice is to always remember that quilting is supposed to be fun and relaxing, not perfect!
Things I wish I had known when I started:
No matter how pretty, do not buy cheap fabric. By this I mean thin, or loosely woven, not price-wise. I've had fabrics from quilt shops I would not use, and have even found a few gems at Walmart. It takes the same amount of time to make a quilt with decent fabric as it does with low quality fabric, but your quilt will be much more durable. And don't skimp on the thread, either.
When you find a fabulous fabric that you just must have for your stash, buy at least one coordinating fabric to go with it. I have some fabulous fabrics in my stash that I don't use because I have nothing else that matches. Better yet, avoid accumulating a stash of random fabrics to begin with.
Don't let your scraps pile up. I had boxes and boxes of scraps. For the last two years, one day a month, I spend an afternoon doing nothing but cutting scraps trying to work my way thru boxes filled with 20 years of scraps, and I still feel a little overwhelmed. Bonnie Hunter has a great scrap cutting system, and oodles of quilts you can make once you have built them up. Nickel Quilts are another great use for 5" scraps. I also have made a few scrappy backings out of nothing but 12" and/or 6" squares. Or, use use the scraps to make coordinating throw pillows or pillowcases or table runners, or attach a new border to a set of sheets to make a coordinating set.
If, for whatever reason, you're not thrilled with a quilt you've finished piecing, quilt it anyway. Use it as a backing, as a picnic quilt or an emergency quilt to keep in the trunk of the car, give it away, just don't put it aside. Some of my least favorite quilts have been the most loved by the people I have given them to. Even ugly quilts are warm!
Avoid quilting on demand. I can't count how many people have flippantly asked me when I am going to make them a quilt, mostly by people who I know will not appreciate the time, effort, and expense that goes into a quilt. Plus, nothing is fun when it is expected of you. I tend to dread working on a quilt if I am on a deadline. Only make quilts in pattern and designs that bring you joy, and give them to those that your heart tells you that they belong to.
Learn to accept compliments, and never, ever point out your mistakes when showing off your quilts. As artists, and quiltmaking is an art, we are our worst critics. Most people will never notice if your seams don't line up perfectly or your corners are slightly lopped off. Those that are so persnickety as to look at your work that carefully, don't deserve your quilts anyway! Do your best right now, and know that your best in a year will be even better. My son just brought me one of the quilts I made early on to repair, and I saw so many mistakes in it that I would not make now. But he has used it and loved it for over 15 years! My other son asked me if he could have the first quilt I machine quilted, even though the quilting ranged form poor to mediocre, because he said he was proud of me for learning something new.
Change needles and rotary blades regularly, and thoroughly clean out the dust from your machine regularly. Do not sew over pins. Store your cutting mat flat. Basting spray, spray starch, color catchers and glue sticks are godsends.