I need some help with sentence grammar and I hope you can help me.
What is your biggest goal in life? Is it to write a book, find a fulfilling job or find a love of your life?
Can I start second sentence like this? ("Is it to write....")
Your second sentence is fine
Is it to write a book, find a fulfilling job or find the love of your life?
is a combination of
Is it to write a book?
Is it to find a fulfilling job? or
Is it to find the love of your life?
You can start your sentence it with "Is it...?" since it is a question. However I would change "a" to "the".