The distance from Tullamarine Airport to Melbourne city center is approximately 23km. To reach the city, you have several options: you can take a shuttle, rent a car directly from the airport, book a cab for around AUD 30 (cab drivers usually don't expect tips, just round up), or use public transport.
If you're looking for the most budget-friendly way to get to Melbourne, experienced travelers often recommend the red Skybus shuttle bus. It can take you to the city's business center for just 19 AUD, and these buses run approximately every 10 minutes. You can conveniently purchase your ticket in advance via the internet.