As I mentioned previously, three things are involved in learning how to take pictures with your camera in manual... which in my opinion is the ONLY way to get a picture to look the way you want it to look. One of those key ingredients is aperture. Aperture is basically what is in and out of focus. So, if you like the look of the background being out of focus, you will want to shoot "wipe open" as they say, or in other words, with a low aperture number. What that will do is help you create an image with the subject in focus and the background out of focus. You need to make sure you are focusing on the subject, of course. :) Something else you must remember is that aperture is just one corner of the "triangle". If you are properly exposed (you have the right amount of light coming into your camera so that your SUBJECT is lit nicely) and you mess with the aperture, it will "pull", so to speak, on one corner of the triangle. If you change your aperture, you will need to adjust your shutter speed and/or ISO to keep your triangle balanced.
In the picture above my aperture was set at 1.4 (you will often see it like this: f/1.4). That is as wide open as that particular lens will go. Aperture is based on the lens, NOT the camera. (Shutter speed and ISO are dependant on your camera, unlike aperture). If you look closely, you can tell that I focused on the subject's right eye (my left). It is clear and crisp. Because I shot so wide open, even his left eye and ear are slightly out of focus. You also can't tell what is in the background (which in this case is a good thing :)). The red arrow is pointing to the arm of the couch that is he sitting on. You can't even really tell that's what it is.
Shooting wide open is a great thing... as long as you aren't shooting TOO wide open. People seem to think "the wider, the better". Not always. In fact, for me, rarely is wider better. You have to think of what you want in focus. I like my entire subject's face in focus. Both eyes. Mouth. Stuff like that. So, I generally stick around 2.8 or tighter. I DO like the background out of focus. Especially if you have nice pretty light back there. Bokeh is a beautiful thing.
You have to think about pictures you are taking of two or more people as well. If someone is standing slightly in front of the other person, you need to take that into account when setting your aperture. If I'm taking family pictures I NEVER shoot wider than 5.6. (Only made that mistake once.) So, my aperture number will be 5.6 or higher. You want to make sure that everyone is in focus. (there are, of course, exceptions.) I know if I was a client there's no way I would pay big bucks to have a fancy picture of my family if two of my kids were slightly blurry.
Ok, in this picture I focused on the same spot (can you tell he was annoyed that I was taking time away from the Wii?) The difference in this picture than the previous one is that I changed my aperture to f/13. That's much tighter therefore much more will be in focus and there is not nearly the amount of light coming into my camera as when I was shooting wide open. Because I tightened my aperture, I had to adjust my other settings to make sure he was still properly exposed. I changed my shutter speed slightly but most of the change came from the ISO. It went from 250 in the first picture to 4000. BIG change. But, that's what I had to do to make sure he wasn't nearly black. You can also tell much better what is in the back ground since I shot tighter (remember, more in focus). Obviously, both eyes are not in focus. There's a wadded up purple Tinkerbell blanket just past the arm of the couch. And who can miss the hubby's drying hoody? (laundry day.) Big difference from the first picture, right? In this case, I like the wide open shot better. :)
At the end of the day, aperture is a powerful tool. It is so much fun to play with and can completely change the mood of your image. Play around with it. You'll be shocked at how much more you'll like your images once you have a better understanding of it. Good luck!