Taxi options are black and white…. literally. White taxis can pick up fares anywhere and should use the meter (although some drivers still refuse to do so – their ‘fixed prices’ are often inflated).
Black taxis may not cruise for fares - they need to be booked in advance by phone or online but are often cheaper than white taxis. There are taxi ranks in Valletta at the City Gate and Grand Master’s Palace, as well as in the main tourist resorts. Taxis from the airport operate on a fixed tariff (for example, the airport to Valletta costs €15; to Sliema, it’s €20 and to Cirkewwa, it’s €35). There are also fixed fares from the Valletta Seaport and a ticket booth in St. Julian’s.
Uber is limited in Malta, but there are lots of traditional taxi companies which you can use to get you to your destination, ranging from more corporate transport to eco-friendly cars. Most offer a fare calculator on their website and some also take online bookings. They offer “low pick up time frames from any point in Malta”. There is also Taxify - a local company which is a very similar service to Uber.
Public transport or getting your own car may be other alternatives you may want to consider.