Help on measuring temperature

Hey, I'm new here. Just wanna ask, if I were to connect my Arduino Uno to measure temperature, what exactly do I need to do? A friend of mine told me through chat that I may need a PT100 Platinum Resistance Thermometer or thermocouples. Which one is better? What else do I need? Or are there any better suggestions? The temperature I'm about to measure is steam at 200-400 Celsius, under pressure.

Thermocouple it is - these are most commonly used though I imagine a Platinum Resistance Thermometer would also serve.

You will need a thermocouple amplifier - best go for one ready made - as indeed you will be using thermocouples from the same supplier.

Beyond that, "Google is your friend"!