At 18 what would be the best first investment for the beginning of a entrepreneurial career?

Investing comes after you make money. Unless you are independently wealthy.

My business came before 99% of my investments.

Investing is not being an entrepreneur...just saying...

How much money do you have?

How much money do you have to invest? What do you know about and are interested in?

There is no one correct answer. It depends on things like: What is your timeline? What is your business? How much money do you need to start your entrepreneurial career?

Most financial advisers will tell young investors that their first $10,000 should be placed in a broad index fund or ETF like the S+P 500. Then based on your risk tolerance and timeline you make investments in the stock market.