You have 2 options.
The simpler option is just using the 3.3V power as the analog reference. You get a measurement range of 0 to 3.3V. The downside of this approach is any variance or noise on the power line impacts your measurement. For many uses, this is good enough.
For higher accuracy, a voltage reference chip like LM4040 or LM336 is used. You want a "shunt" type voltage reference. Often they are shown with a zener diode schematic symbol, because they work like a zener. Inside is a complex circuit which emulates a zener diode, but compensates for temperature changes and other effects so you get a very stable voltage. The measurement range becomes 0 to whatever the chip's voltage is (must be less than 3.3V). This approach gives superior results, depending on the accuracy of whatever chip you buy, but of course the downside is you have to add another part.
The really accurate voltage reference chips are expensive. Some people buy a cheap one that is stable and use a high quality voltmeter to measure it and store the actual voltage as a calibration constant in their code or EEPROM data. This works well, but requires the extra work to measure each individual chip and store the data.
