Jepara is a city in at the north coast of Central Java. Jepara is known as center of teak-wood furniture production, the birth place of the national heroine Kartini and the woodcarving industry. Jepara is the jump of point for ferries and speedboats to Karimunjava.
From Semarang there you can hail a taxi and bargain a price of around Rp. 275,000. Even if they use the Argo (meter) it should not be much more than that.
A short visit to the TIC or 'Tourist Information Centre' in Semarang will ensure you find transport to Jepara, usually a small comfortable bus seating around 10 people with air conditioning. They leave at multiple times daily, for around Rp. 30,000pp.
Buses from Semerang leave Terboyo bus terminal everyday from 5am to 5pm and cost between Rp. 10,000 and Rp. 15,000. Expect to pay twice the amount if you put your backpack on an empty seat (there are no luggage compartments on small buses).
Taxis are a bit hard to find. Becaks are fairly easy. Motorbikes can be rented from some of the motorcycle dealers.
Schedule for ferry service to/from Karimunjawa Islands is available at Karimunjawa Info Page.
If you know Indonesian You can call Raja which is a taxi service using a normal SUV type car. 085 225 111 234 or 081 705 62002 You can also call Atlas taxi in Semarang. If you don't speak Indonesian try going to Jepara Indah Hotel, or Kencana Hotel. The Hotel Sugoro on the North end of town (near the gas station) has a travel service and can also charter cars. There is a bus station in Jepara also with buses to various locations including Jakarta and Surabaya.