Your best topic is always a subject you have passion for, knowledge about and experience with. Your personal life experiences have taught you a unique set of lessons. Determine what you know that others might find valuable.
Of course, you can make a sales web Site that focuses on almost any subject you want but you will have a better chance of success, especially if this is your first site, if you choose a subject you have some experience with.
Your product is the information you have that other people will find valuable. Personal experience is the best source of real knowledge. If you consistently draw on your experiences to produce your content you will create information that other people will find useful and will be willing to pay for.
Plus it makes the process much more fun to write about things you understand and love.
The best place to start is by brainstorming a list of topics. Get a pad and pen.
What are you passionate about? Write it down. What do you love to talk about? What do you read about? What specialized knowledge do you have from your job? Any hobbies? Start writing. Get everything down. Dont censure anything,...