You can use both start and begin for an activity. “Begin” is more formal than “start”:
▶️ I started playing the piano when I was 8 years old.
▶️ What time does the meeting start?
▶️ He’s beginning to read more advanced books in English.
▶️ We left the park when it began to rain.
When you turn on a car or vehicle, use “start”:
▶️ I had to call a mechanic because my car wouldn’t start.
In general, begin is used for more formal and more abstract ideas:
▶️ Scientists are studying how life began on earth.
▶️ World War II began in 1939.