Now that you have got your bootable media, you need to boot from it. For this you need to reboot your computer, so save all your work and reboot. Now is also a good time to put your bootable media into the computer.

When the manufacturer logo appears, you need to enter the Boot Devices menu by pressing a key. Unfortunately this key is different for various computers, so you will have to see if it says what it is near the logo, or by searching the internet for your specific hardware. The most common keys are: Esc, F10, or F12.

Then simply choose CD/DVD or USB KEY (or something similar - it's pretty obvious) depending on what you want to boot into. After a short time you should see a screen asking you if you want to Install or Try elementary. Feel free to try it, but for this tutorial we are going to click Install. If your main language isn't English, be sure to pick it here too.

On the next screen, three checks are done: If you are connected to the power, If you are connected to the internet, and If you have at least 2.6GB free disk space. The latter is the only compulsary one, but if you run out of power during an install, you could lose all of your files, so make sure it's plugged in.
On this screen there are two very important checkboxes. The first will make sure your copy of elementary is fully up to date right after the install, and the second will install plugins for MP3s and such. Don't miss these checkboxes!