This mostly has to do with something called tension. Most likely what you are doing is holding your working yarn too tightly as you are knitting. This is one of those things that just comes with time and practice. It takes a while to get used to holding the yarn properly and letting loose when you need to and grabbing tighter when you need to.
Also what people tend to do is get a grip on the yarn and keep holding it tightly as they knit. So what was slack three stitches ago is now tight becuase you haven't adjusted your death grip on the yarn in between stitches. You'll notice when you first start knitting that you are going to be so afraid that a stitch is going to drop that you will hold your needles and your yarn so tight that you will give your hands cramps. You don't want to be a white nuckle knitter. Just relax and take a breath and check in every now and again to see how tightly you are holding things. You'll notice at first that you start out relaxed and get very tense as you knit. It's just the stress of learning something new working its way in. Take a moment when you notice this to breath and relax.
I find that I go back and forth between holding the yarn and releasing the yarn. While I am actually makeing the stitch I hold the yarn the tightest that I hold it, but I'm still not gripping it to the point where it doesn't have any give. Then in between stitches I loosen my grip a little to allow some extra yarn to come in to replace the yarn that I used making the last stitch. The way that I do this is by wrapping the yarn around my pinky and then I just squeeze my pinky in when I need more tension and let my pinky relax out when I need less tension.
Don't get discouraged by the fact that you are having trouble right now. It takes time and lots of practice to get this stuff right. I know there are still things that I pick up and I'm sure there are tons of things I have yet to learn. We all started where you are and we all had the same problems you are facing and we all got through them and learned to be better knitters. Just take some time to practice the basics before you start working on something you're going to be upset doens't turn out right. I practiced making garter stitch pot holder and wash cloths when I started. It's just practice and who really cares if your wash cloth is a little wonky.
OTN: Jeckyl and Hyde Hat, Shetland Shortwrap( )
FO: Lacey Stole, Library Capelet, Dead Fish Hat, Tree Bark Scarf