You can get to Djerba by public transport - minibuses or public buses. The one way trip will cost approximately 20-25 TND. Lojazi depart from the bus stations as they fill up, with a bus schedule available here.

From Hammamet it may be more convenient to get to Tunis, from where you can get to Djerba by airplane. The approximate ticket price is 100 TND one way. The schedule of flights can be viewed here