Here are some examples:
Capitalized:
* I checked the Internet for information about the event.
* She works in the Internet industry.
* They connected to the Internet using a Wi-Fi network.
Not capitalized:
* We need to improve our internet speeds.
* The new internet service provider is offering a great deal.
* They are working on expanding internet access in rural areas.
Think of it this way:
* "Internet" is like "Earth" or "Amazon River" - a specific, unique entity, and therefore capitalized.
* "internet" is like "water" or "electricity" - a general concept, and therefore not capitalized.