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?
Above user just for troll posted the reply without reading the OP question, he is asking to earn freely without investment. Only way is to just go through forum fully and search which ever work is suitable for you to do and earn it, their are lot of ways to earn free bitcoins it is only that we have to put some effort to earn it, right now i am earning nearly 0.3 btc per month. but very soon i will able to earn 1 btc in 2 months frequency