Edit: first check that you're not on a two second delay. Push the set button, then the left button. That will tell you what shooting mode you're in. I'd keep it in continuous unless you need something else.
It could depend on a lot of things. Is it making noise like something is moving? If so it's hunting for focus. This could be either because your in dim light, or trying to focus on something closer than the lens will allow (the minimum focus distance will be printed on the ring on the front of the lens). Some lenses are just slow to focus too, so if you were in focus for something far in the distance, but then try to focus on something close it can take a second. But if it's taking a few seconds I'm guessing that it's one of the two things I mention above.
Or you're just trying to focus on a blank wall and it can't find a contrast point to focus on (at least one of those little red dots in the view finder need to be over something with contrast to detect focus). Just to be sure you're on the right focus mode, there's a little checkerboard symbol next two one of the two buttons on the upper right of the back panel (near your thumb). Push it, and see that it lights up all the 9 points or so of the autofocus array. If it just has a single point then move the arrows around until you get all of them.
