Unless you are a total bad ass like DeJesus, you'll probably want to have some vague layout of your figure before you really start putting in any details. I do this by drawing a circle for the head, and from that I draw a curve that represents the spine. Then I draw in a second ellipse for the chest and further down at the end of the spine, a third ellipse for the hips. The eyes should be halfway between the chin and the top of the head, so it helps to draw a horizontal line here to get an idea for where the eyes will be placed. I like to draw a perpendicular line through that to mark the center line of the face. This helps me visualize where the face will be looking, and it's good for keeping things symmetric once you start laying down facial features. By this point, I have something like this:

Next, I draw in a neck and two concave curves to connect the chest to the hips. Arm and leg placement and length can be tricky, it often helps to mimic the pose you want to get an idea for how long the appendages should be. Usually I draw circles for the hands and elbows before I draw the outlines of the arms, because it's easier to keep track of the global picture. The same goes for feet and knees, although in this case I avoided that. Fingers can be difficult to draw, again it helps me to mimic the pose I'm trying to draw. My picture now looks like this:

Now before you continue, stop and hold your picture out at arm's length and look at it. Turn it upside down and examine it, and look at it reversed in a mirror. You may even want to scan the picture and perform a horizontal flip to look at it. Looking at your picture in different ways can often help you catch errors before you get too far. You get used to looking at your art a certain way, and having other views of it can bring out the errors you have difficulty catching. I used to have problems with causing characters or the flow of the picture to lean in a way that didn't look good, and I had trouble catching this problem until after the image was inked.