Formentera doesn’t have airport, so it is only accessible by the sea.
If you don’t come by your private boat, then you will have to arrive first to Ibiza and from there take a boat or ferry to Formentera.
During the summer months, there are also direct ferries to Formentera from Barcelona, Valencia and Denia.
If you fly to Ibiza, the boat to Formentera takes around half an hour, and leaves every half hour in the summer. You can buy the ticket on the boat, and there is no need to reserve, but if you plan to come in the high season with your own car, then it is better to reserve your ticket in advance.
There are many local companies offering boat service to Formentera, covering the route every day with different frequency depending on the time of year.