The general idea is that you must get this holy trinity in balance in order to get pretty pics, based on the amount of light able to get into your camera through the lens. (Of course, there are always exceptions to the rule.) Luckily unless you're shooting on a really old camera (I'm talking before digital was invented) there is a nifty little thing called metering that helps you work this out. That's the bar that looks like that thing below.
If you are shooting on a DSLR you probably have a spinny wheel on the top or side of your camera for adjusting this (as well as a button that looks like this: +/- that helps you switch between them.) My camera also has a separate button for adjusting the ISO. If you are shooting on a bridge or point-and-shoot, you might have to click arrows in order to adjust them.
I added a new lens to my camera (a 50mm Canon lens) which gives me a super low aperture (or 'F - Stop') in order to get super blurry backgrounds of wonder. You can go really low, such as F/1.8, that makes it super soft, however it is sometimes better to shoot a little higher, like F/2.8. It gives the same effect but gives a little more clarity to your lipstick, cake or whatever it is in the front of your picture. If you don't have a lens like mine, your aperture might not be able to go down as low as mine. Usually, the one that came with your camera will be able to go to something like f/3.5, which should still create the same effect, to a lesser degree.
Below you can see the contrast between the 'low' and 'high' aperture settings using these fantastic shoes for an example.The bigger the number the more of the image we get to 'see'. (you can see that there is a lot more detail in the fur in the background of 'high' and more of the other shoe is in focus) Again, check out my hydrangeas in front of the window:
To me, aperture is the most important part because it defines how your image will look. For my outfit photos, Martin keeps the aperture low for close ups (pretty, crisp pictures of handbags or whatever) and raises it for full body shots to make sure that I don't get them onto my computer and realise I'm half blurry. ;) To change the aperture on my camera, I have to hold down the (+/-) button and scroll the wheel on top of my camera to raise or lower the number.
Following on from aperture, we get to shutter speed. This is the next step I take when composing an image. If you check out the 'metering' bar, you will probably see that the arrow on it moves around when you move through the aperture numbers. The idea is that you keep the arrow as close to the middle as possible, with a few bars leeway either way. If you're raising the aperture, the arrow is likely to move to the left. This means your image will turn out too dark, moving to the left will make it too bright and washed out. For this reason, I generally keep my aperture low, but depending on your light, you are probably going to have to compensate for your choice of aperture with shutter speed.
The lower the speed, the blurrier the photos. This isn't always a bad thing, for instance: spinning rides at a fairground, a racecar zooming past, fireworks leaving trails of light behind them, car headlights zooming around a city. For these you either need to be pretty good at standing still, or own a tripod.
According to Martin's tutor at Trent, a super professional photographer learns to keep a camera steady at "1/60" (this is how shutter speed is displayed on your camera - One sixtieth of a second) in his bare hands. It's safe to say that I keep mine between 80 and 100, in order to to make sure my pictures aren't blurry. When it's super bright, you can raise it up much higher to compensate that metering, but I generally don't go below 80 to avoid blurry photos. Again, if you cycle through the shutter speeds, you will see that little arrow move around. Again, the goal is to get the arrow pretty close the the centre. To adjust shutter speed on my camera, all I do is have to move my little wheel on the top of my camera from left to right.
Finally, onto ISO. ISO does two things, first of all, it makes your photos lighter or darker, helping to compensate that little moving metering arrow, This sounds like a great thing, so you should use ALL the ISO, but it sadly can also add grain to the pictures.(By grain I mean think of how crappy your iphone pics look in your bedroom at night compared to in the day.)
The higher the ISO, the more light, but the more grain. The lower the ISO, the more 'crisp' and grain-free the photo is, but less light. For this reason I aspire to keep said setting at 100, which is the lowest it will go on my DSLR. Of course, life isn't always that easy and sometimes I have a super low aperture (remember, more light) and the lowest shutter speed I can achieve without blurriness, but that arrow is still not really hitting the middle and my photos will still come out dark. (This is usually when say, the sun is setting or I'm in a room with generally less light.) In this case, I will up the ISO in order to help my camera suck in some more light, at the expense of a bit of clarity. I say this, but usually my photos can comfortably go up to 800 or maybe even 1600 without looking awful, but the general aim is to keep it low.
Hopefully, all that rounds it off quite nicely! I hope I wasn't TOO patronising, I was just trying to come at it for someone who knew nothing beyond the Auto setting. I actually loved writing this so let me know if it actually made any sense/if you learned from it, and if you have any questions I will do my best to answer them! You can comment down below and I will strive to answer, but for a probably more prompt response, you can tweet me @anniepancake!