This depends on your coin of choice. It's unlikely for most altcoins that you can exchange them to regular currency directly or buy anything.

The most general way is if coin is listed at exchange - you can exchange it there to BTC/ETH and buy something with those or exchange them to USD, your options for exchange to USD depend on your country of residence.

Some coins have in-built ability to be of use in the developers' system or already have shops where you can buy with them directly and/or exchange to USD.