The most utilized type of diffuser for oils today is an ultrasonic diffuser. This type uses ultrasonic waves and water to disperse essential oils into the air.
Most diffusers have a water level line on the inside of the water reservoir or a measuring cup to fill up and then add to the reservoir, refer to your diffuser manual for the recommended water amount your diffuser requires.
Add 3-10 drops of essential oil or fragrance oil to the water (you can experiment with the oil amounts to see what you prefer). You can use a single oil like lavender, or an oil blend.