Valletta is a small city, so most places can be easily reached on foot. However, if you prefer to take public transportation, buses are available at reasonable prices. A one-way ticket costs €1.50 and is valid for 2 hours from the time of purchase.
If you have more time, you can explore other historic sites in and around Valletta, such as the Fort St. Elmo or the Malta Experience. If you have less time, consider skipping the Casa Rocca Piccola or the Grand Master's Palace. Don't forget to wear comfortable shoes and bring sunscreen, as Valletta can get quite sunny and hot.