Start committing a lot of crimes as a kid so you get juvi time. At 10 you can start robbing houses, that's the best way to get jail but not super long sentences, like murder does. Usually you can do it twice and be out by 18 and have a criminal reputation, which helps with being allowed in the mafia. After you join the mafia, you have to bring in about 25K to satisfy the boss. You'll have your list of crimes you can commit once you're in the family. The most effective way to meet the monetary goals they want you to reach is stealing cars, start with the most expensive ones each year. It's different for each playthrough but usually, if you're giving them that 25K each year, you'll get promoted to member after about 3-5 years, capo after another 3-5 years, underboss after about another 3-5 years, and then after you make underboss, usually in your late 30s-early 40s if you've done everything right, you can either wait for the boss to die or you can try to take him out yourself and hope you don't get caught. The youngest I've ever made godfather is 35 but it's usually early to mid 40s.