I have a fairly wide variety of friends. Here's some advice I find myself giving often, because it seems to cover a lot of what I think are the most common problems. The wording here isn't how I'd say it to them.

Health and lifestyle

• Don't engage the services of any non-evidence based medical practitioner.
• If you have a health problem, you're receiving treatment/advice, and not obviously improving, get a second opinion. And probably also a third. (I am not in the US)
• Don't smoke cigarettes. If you already smoke cigarettes, buy an e-cigarette (like, right now), even if you're sure you'll never use it. Once you've bought it, do try to use it, even if you don't think you'll like it.
• Always use barriers for vaginal and anal penetration outside a monogamous relationship
• Use either barriers or hormonal birth control for p-in-v intercourse
• If you or your partner is having non-monogamous sex, get STI tests twice a year
• If you frequently feel depressed or anxious, see a psychologist who practices cognitive behavioural therapy. Do not see one who practices psychoanalysis (see "evidence based health care"). Expect to trial several psychologists before finding one who is a good fit for you (see: "if health care not working, seek second opinion").

Personal finance

• Don't buy cars new. There's an established value-point for depreciation vs maintainence costs for cars. Use that.
• Don't get a credit card
• Don't take personal loans unless the interest is dominated by the cost of having no liquidity (e.g. eviction)
• If you can't maintain a monthly budget, give yourself a weekly budget. If you still can't maintain a weekly budget, give yourself a daily budget. A budget over a shorter time period is much less convenient --- but failing to maintain a budget at all and going broke is less convenient still.
• Never gamble, including playing the lottery
• Don't invest in individual stocks, or pay someone else to invest your money in individual stocks.
• Comply with your local tax laws, including filing your taxes on time. Know that people smarter than you are being employed to catch tax cheats who are also smarter than you. You will likely lose.

Career

• Avoid "winner takes all" professions, e.g. sports, music, academia in fields with no industrial application, etc
• Judge potential careers by expected value, not best or worst possible outcome. Look at median salaries and working conditions.
• Only enrol in tertiary/graduate education with a specific career outcome in mind
• Recognise the true cost of tertiary degrees, in opportunity cost as well as course fees. e.g. a full-fee-paid PhD program with a modest stipend is still enormously expensive
• Recognise that reputational capital distorts labor markets. Teachers are paid low because they're well respected, not despite being well respected. The how-do-you-do value of having a well respected career is not that enduring, so it's probably over-valued. It's better to avoid careers with excess reputational capital.
• Consider careers in the allied health professions . These professions have the best job security: the aging population ensures rising demand, they must be performed locally, and most seem like unlikely targets for automation or disruption. They also offer lots of human contact, which many find produces good job satisfaction. But, because they don't require medical degrees, they are fairly neutral in reputational capital, so your peers don't just work themselves to death to win zero-sum positional games against you.

Law and the justice system

• Avoid dominance contests. Learn to display submission readily, meekly and convincingly.
• Regard police officers as people who have power over you. They do. Their power is broad, and not constrained by some set of written rules. If you challenge them to a dominance contest, you will likely lose. They will fuck you up. And even if you "win", it'll be a pyrrhic victory --- you'll still have been better off not getting into it in the first place.
• Power is not moral authority. Someone may have real power over you, with no legitimate right to it.
• Moral authority is not power. Even if you have the legitimate right to do something, someone may have the power to stop you.