The best and honest way is to accept Bitcoin as a form of payment for the goods and/or services that you produce.
Become a millionaire in BTC?
That is very hard and it's not a realistic goal.
Remember that there will only be 21 million BTC total, so theoretically, there can only be 21 maximum millionaires possible....
for the whole world?
Forget it.
They are talking about becoming millionares on fiat, is there is the case that each bitcoin worths 10 million usd (in 2050 when bitcoin is already in 4 planetary systems), so you can be a millionare with 0.1 btc.