As Formentera has (fortunately!!) no airport, the only way to reach it is by sea from Ibiza island (all year) and directly from Denia (Spain peninsula), only in summer.
All passengers carrying routes to Formentera operate from the port of Ibiza Vila.
There are several companies that cover the Ibiza-Formentera crossing, that usually takes between 25 (for fast ferry) to 50 minutes (ferries carrying cars). These companies offer a wide choice of timings, schedules, offers... so it can be a good idea to check some of them before buying your tickets. These companies are Balearia, Mediterránea Pitiusa and Trasmapi.
In order to arrive to Ibiza there are regular services from the ports of Barcelona, Palma de Mallorca, Valencia and Denia. The other way to get to Ibiza is by air, and here you can find a wide range of possibilities depending on your country of origin.