When you give with one you take with another to ensure that you don’t over- or under-expose your shot. In your early days of setting your exposure manually, it can be a bit tricky to remember how they fit together. But if you think of exposure as a glass being filled by water from a tap, it might help. Here’s how it works.
Think of aperture as how much you open the tap. The wider you open it, the more water flows into the glass. Open the aperture wider and you let more light into the camera.
If aperture is how far you open the tap, shutter speed is how long you open the tap for. If you open it for a long time, a lot of water comes out. If you open it only briefly, just a little water comes out.
The final control in the exposure triangle is ISO. From the perspective of our water-glass analogy, ISO could be described as the size of the glass. At a low ISO, the glass is a large one. At a higher ISO, the glass is smaller. In effect, if you are shooting with a high ISO, you need less water to fill the glass.
Much like filling a glass with water, getting an exposure can be achieved quickly or slowly, but you’ll get there in the end. You can use a trickle of water—so that would be a small aperture—for a very long time, and the glass will eventually be filled. Alternatively, you can open the tap all the way—the equivalent of a large aperture—very briefly, and the glass will be filled.
The same is true in photography. Two different exposure settings can result in the same amount of light being recorded inside the camera.
How, then, do you put this into practice? It depends on what you want to achieve with your photo. If you’re looking to secure a particular depth-of-field you’ll set your aperture accordingly and then adjust the shutter speed and ISO to complete the exposure. For a long exposure shot you’ll need to think about whether it needs a small or a large aperture and a low or a high sensitivity.
More unusual ways of looking at things and remembering rules are in my lovely book, The Rules of Photography and When to Break Them. Available as an e-book and in a dead tree version (UK, US).