There are a few different types of thermometer to know about. Perhaps most common nowadays are digital stick thermometers, which are simple, easy to read, and can be used for oral / rectal / armpit readings (check the notes on your particular model).
Smart thermometers are app-connected, to let you track temperature over time, and can be useful if you need to spot patterns and share info with your GP. No-touch thermometers will let you check temperature without the need for actual contact with the skin. There are thermometers specifically designed for ovulation tracking. And some of the best thermometers aren't for checking body temperature at all: room thermometers let you keep an eye on, for instance, how hot or cold it is in a nursery.