There are many out-of station transfers that require you to pay another fare, to get where you are going. However, if you put some effort into it, you can avoid taxis and not walk too much.
An easy way to cope with the system is to download city-smart applications such as “trafi”. Google maps also show updated transport options between two points of travel.
There is an extensive bus system, including city-run and private buses, as well as one high-speed Metrobüs line; an extensive light rail system including six Metro (underground) lines, four Tramvays (aboveground), three Fünikülers (ascending/descending), two mini-lines called Teleferik, and the Marmaray (underwater) lines; and the ferries which travel the Bosphorus.
Each time you use a tram, metro, bus, or boat on the public transport system, you will need to touch a magnetic card known as “Istanbulkart“. You can purchase your personal card at one of the yellow vending machines found at all major metro and rail stations. As of 2021, an Istanbulkart costs 10 lira. You can then top up your card at any yellow ticket machine.
Note: except for some of the latest ticket machines, only cash is accepted.
Most stations have an ATM. Ticket fares across buses, trams, and metros are at a flat rate (i.e. not dependent on how far you go).
Also, be aware that the Istanbul subway system does not offer transfer tickets and as such each new line requires a new fare unless you use Istanbulkart.
See the dedicated Istanbulkart website for information on all the cards available and discounts.