This is a very common problem for newbies. Nothing that can't be fixed by a couple of things. The first thing to notice is that most patterns will say to use a certain size hook (OR SIZE TO FIT GAUGE). This means that if you have crocheted the proper amount of stitches for the gauge, yet your swatch is quite large, you'll need to crochet with a smaller hook. If your swatch is smaller than the swatch should measure, then simply change to a larger hook.
As you crochet more, you'll learn to get your tension more even. But, everybody crochets different. For example; I'm considered a tight crocheter and my sister is a loose crocheter (there's no such thing as a perfect crocheter). You will be either a tight or a loose crocheter. That's just the way we are made. Although I find that most people are loose crocheters, that doesn't mean that you're wrong if you are a tight crocheter. Just adjust your hook.
Another thing is that if you find you crochet too loose and you'd really like to crochet tighter, there's a simple technique that will most likely help you. But, first let me explain what makes a tight/loose crocheter.
It all has to do with how we hold the yarn and how often we move our hands "up" on our project as we work it. What I mean is this: as I'm crocheting, I'll crochet approximately 5 sts if I'm using yarn and 8-10 (depending on the size of my hook) if I'm using thread. The reason for this is because I like to keep my fingers close to the hook as I'm working. I feel that this makes my sts more even. Another reason I do this is because I have small hands and short fingers. Therefore, I'm not able to keep holding onto my project, at the original point, as I'm crocheting and get too far away from the hook. I end up running out of hand space.
My sister, who crochets loose, I've noticed will hold her yarn different as well. While I generally wrap the yarn around my pinky, under the next two fingers and over my index finger, my sister will simply hold the yarn between her index and middle finger. Therefore, she has less control on her tension that way. Her hands are also somewhat larger than mine and her fingers a bit longer.
The good news is that about the only time a gauge is important is when you're make articles of clothing that must be fitted. Otherwise, does it matter if your dishcloth is a bit smaller or larger or that your scarf is longer or wider? It doesn't matter to me.
And remember, the longer that you crochet, the more apt you are to teach yourself little tips and tricks along the way that will benefit yourself with your crocheting. You'll find new ways to make a slip knot (They all turn out to be a slip knot, but there are an indefinite number of ways in which to accomplish it), new ways to hold your hook and yarn, various methods to stitch two seams together and many other useful bits of knowledge.
I do hope this helps you in the future so you'll feel more adept at the wonderful fiber art of crochet.
