The Tourist Information Centre should be your first stop if you're interested in any kind of city tour. They offer a variety of scheduled tours for individuals, and can arrange even more for groups with a bit of advanced notice. Prices and itineraries are also clearly listed on their site, so you'll know exactly what you're getting. Almost every hotel in the city offers guided tours of one sort or another as well. Some are good, others just a way of making money. You should be able to spot the scams without too much difficulty. Most tickets can be booked online or purchased at TIC (Adamič Lundrovo Nabrežje 2).