Your aperture is how big the faucet pipe is--the size (or how open/closed it is) determines how much water can get out at once. A wide aperture (lower numbers like f/2.8) is a big hole and lets in a lot of light. A narrow aperture (like f/16) is a very small hole and lets in minimal light.
Your shutter speed is how long the faucet is left open…aka how long the water is allowed to flow. More time means more water gets through. Less time means less water gets through. Having the faucet open for 1/200th second lets in less water than having your faucet open for 1/60th second.
Lastly, the ISO is the size of the bucket. A big bucket is a low ISO like ISO 100…it takes a while to fill up. A small bucket is a high ISO like ISO 1600, it fills up quickly. Long story short-- a higher number ISO is more sensitive to light/water.
Now you have an understanding of how to use aperture, shutter speed or ISO to add more or less light to an exposure. Your goal is to vary these elements and get a perfectly filled bucket--right up to the top without overflowing. You must do a little juggling act…when you set one piece of exposure in place you must figure out how can you vary the others to get just the right amount of water (aka light).
If your photo is too dark, you can use a wider aperture, higher ISO or longer shutter speed to let in more light. If your photo is too bright, you can use a narrower aperture, a lower ISO or a shorter shutter speed to let in less light.
Your camera actually has exposure modes that help you achieve this balance more easily. Aperture-priority (Av) allows you to set your aperture, and the camera adjusts shutter speed appropriately. Time Value (Tv) aka Shutter-priority allows you to set your shutter speed and the camera adjusts the aperture appropriately. Basically, your camera helps you out and makes a balanced exposure a lot easier!