Everyone experiences lag from time to time. Lag is often the Xbox (Microsoft) servers themselves, however if you are always the lag source, then there are several potential issues and/or solutions:
1) Open/Closed NAT (Network Address Translation): Your NAT should be set to open or not restricted. This is not a console setting. You must log into your router to change this setting. Typically this will fix your issue (especially if game chat or joining is impacted).
Additional changes: Note: If you do not know how to make changes within your router, then get help from someone who does (parent, or write back with your router brand and model number and ask for assistance).
2) Within your Router ENABLE the UPNP setting.
3) Another option: Within your router, assign your console a static IP address. Then set up port forwarding for the following ports: 88 UDP and 3074 UDP & TCP. This way you can keep NAT closed, but allow unrestricted internet traffic to xBox.
4) If using wireless network adapter, unplug it and run a CAT5 cable from the Xbox directly to the router. If your lag issue goes away, then there could be interference with the wireless traffic. If interference, then log into router and change the broadcast channel.
5) High speed internet and traffic: You really need high speed internet (cable or DSL), close proximity to the router and bandwidth to fully enjoy online gaming. If there is a housefull of net users (Youtube, movies, music, gaming, etc..) then you can expect lag issues.
Upgrade to a higher speed package. 3 or 8 Mbps download is fine if you are by yourself, but with lots of activity, then you need 16 to 30 speed (which is really volumetric capacity and not speed). Check your download/upload speed at www.speedtest.net
6) Make sure your Modem (not router) is DOCSIS 3 and not the older DOCSIS 2 versions. Ask your ISP to check your connection for any potential problems. They can typically read this from their end.
7) Check all cable connections coming into the house, all the way to the Modem. If DSL, then check phone lines and filters.
8) Finally, and I am sure this never happens, if surfing off of a neighbors system, then stop mooching and buy your own! ;-)
Hope this helps!