what happens if I dont use my debit card?

The debit card does not contain any money. Even if your checks do get directly deposited, they deposit into your checking account, not "onto" your card.

You do not need to cancel your debit card; you can simply not use it.

A debit card is simply a way to access money that's in your checking account--the card itself is useless without a connected fund source.

Much like how people used to primarily pay via check if they didn't have cash, a debit card serves exactly the same purpose: You don't have the cash on you, so you give someone access to a certain amount of it via a card transaction.

Nothing will happen if you don't use the card.

Nothing will happen if you DO use the card (assuming there's adequate money in the account to cover the purchase).

You might have gotten a bank account where there is no fee if your paychecks are direct deposited. If that's the case and you don't get direct deposit, they will charge a fee.