The city center is navigable on foot if you're based in or near Kota Lama. For longer distances, ojek (motorcycle taxis) and becak (cycle rickshaws) are the quickest options. Grab operates in Semarang and is more reliable than flagging down street taxis, which rarely use meters.
The BRT (Bus Rapid Transit) Trans Semarang runs on several corridors and is cheap and air-conditioned — useful for getting between the train station, Simpang Lima, and other central points. Maps are available at the main terminals.