I just bought this plant two days ago, and I'm still confused about how to care for it, especially how often should I water it?
This plant is quite drought tolerant, so you can be pretty easygoing about watering it. For newly planted lavender, just water every other day for the first week. Then, once the roots are established, you can gradually reduce the watering to once or twice a week.
I would recommend watering this plant once every two weeks.
Your English lavender will do really well if you only give it water when there's not a lot of rain, usually about once a week. It's pretty tough and doesn't need a lot of water.