simple, just buy it from an trusted seller
But buy only when bitcoin's price down. You can see now bitcoin in a higher price and i don't think that it is a good time to buy the bitcoin. Also when we buy it in a low price than sell only a high price it need time.
So how can we earn 1 bitcoin into a month in this way?
At this time the price of bitcoin is simply too high the best we can do is to wait for the right moment, it seems to go against logic but we must buy when the price is going down and we must sell when the price is going up that way you can make good money.