It's super easy, plenty of ferries every day from the terminal in Naples. If you're going at peak travel times (summer) it's advisable to buy your ticket ahead of time because the popular times fill up fast. Go down the day before and buy the ticket for the next day.
I was there last summer, caught the ferry from Positano. The main harbour and Annacapri (the town up the funicular) is great, but explore the other side of the island too. Beautiful.
Check out our travel safety tips for Italy here:
https://www.worldnomads.com/travel-safety/europe/italy about 2 years ago