Summarize:

If you're using a wireless connection, you've connected your Xbox to your router, which is the device broadcasting Wi-Fi into your home. It'll be connected to your modem, which is what brings the internet connection into your home. If you know your internet connection is already working with your other devices, this will be most helpful for you. Choose the option to test your Xbox Live connection when prompted and note the number of bars. With 3 or 4, your connection is good enough to not troubleshoot it. One or two bars indicate poor signal strength, and you should continue. Make sure both the router and the Xbox are off the ground, away from walls, and relatively unobstructed. They should be in the same room. For the best connection, place the Xbox and the router next to each other or at least insure there is nothing between the two - draw a line of sight between the two. Be sure there is nothing electronic between the two at any point.
Find your router and Xbox. Navigate to the My Xbox utility and select "System Settings" from the menu. Text your connection. Check the location of your modem or router. Relocate the devices.