can anyone tel me how can one earn from their personal blog

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I am a starter in blogging. I heard that there is a possibility of earning through our blog. can you please tell me what do we write in blog? how frequently should we write blog? how to popularize our blog? though the questions are many, all i want to know is a brief description of this blog development technique. I thank you in advance for your valuable help and assistance. BETTER LATE THAN NEVER..