What are the skills that are most important to learn during your 20s?
1. Build a foundation for health.
In your twenties it's easy to feel like you're invincible. That fast food and the five margaritas you downed the night before burn right off and you wake up the next morning feeling fine. This does not last. Start building healthy habits now before they catch up to you later. At 30, I'm still struggling with this one myself. Get in the habit of eating healthfully and exercising. Your body will thank you for years later.
2. Learn resiliency.
Your twenties is a time when most are relatively free of the responsibilities that will increase into your thirties and forties. This is a great time to experiment, fail, and bounce back. Learn how to ride out failure and persevere. Life is full of challenges. The twenties are a great time to toughen up and start teaching yourself how to be emotionally and mentally resilient enough to weather both the joys and hardships to come.
3. Figure out what lifestyle and career work for the real you.
I felt like a failure throughout my twenties because most of my university friends seemed happy in corporate jobs that made me miserable. I felt constant pressure to fit a specific mold. It's only within the past year that I've started to come to terms with who I really am and what really makes me happy. Life is not one-size-fits-all. Choose the career and lifestyle that are true to you, not a version of that you wish you were.
4. Learn to live within your means.
This point is a related one to the financial advice Jason shares in his answer. Learn smart financial management now. Learn how to live within a budget and maintain good credit. A mistake I once made, and one I still see a number of my peers making, is being a slave to lifestyle: taking cabs everywhere, buying expensive clothing, and having meals out constantly. This is stupid and reckless. I know people who were left completely adrift despite having made great salaries because they had no savings when their careers hit a rough patch. Their income was spent all on things that are frivolous. Luxuries are a wonderful thing only if you can truly afford them. Don't be a slave to funding a lifestyle that will not last. Learn to live modestly and save up, and then you will have earned the right to purchase yourself some treats, in moderation.
5. Choose who you spend your time with carefully.
Time is a precious commodity and you start to realize that increasingly into your mid to late twenties. Choose your friends wisely. Separate yourself from toxic or negative influences, those who would waste your time, and those who encourage you to focus on the unimportant. Seek out people who give you happiness, who inspire you, who support you, and nourish those relationships instead.