This is similar of a question to "What is the meaning of life?", IMO. Everybody's meaning in life is different, and IMO the best thing you can do when you're younger (up to a young adult and beyond really) is to figure out what the hell your calling is. The sooner you figure that out, the easier it will be for you to get into your new life suit and live your best life.
Not only should you figure out your calling, but also what kind of person you are. Are you an introvert? extrovert? Something in between? Do you love having people around? Do you need 2hrs of social interaction a week and that's it? Figure out your ideal balance, and that will help you figure out how to live your best life.
On top of all that you're going to have to figure out a way to pay for everything. If you're fortunate to have been born in or to live in a western country, that becomes a lot easier. Make sure you get a good education and a useful degree. Work/life balance is important, so don't ignore it. You want a career that pays for your needs and leaves you with enough money to pursue your hobbies and other dreams and goals. Don't take on a job where you'll be working 80 hours a week, unless absolutely necessary. Some people will be lucky to end up with a career that's also a hobby.. BUT.. You'll want hobbies on the side even if that's the case. I know people who have gone into a dream career.. 10 years later they're sick of it and not only have they lost a love for what they do but also lost a hobby. Don't end up like that. Work to live, do not live to work. It will eventually destroy you if you try.
Maybe it's just me, but I've found that having a creative outlet in your life is important. Some sort of outlet that allows you to express yourself, whether that means drawing something, writing something, even on a forum.. writing music.. or what have you. Hey, you could build a dam in the woods, by the small stream. That can be artistic.
Embrace your inner weirdness. If you like pancakes with mayo, don't let anybody tell you that you need to conform to the norm. The norm is boring. Conform to whatever works best with who you are as a person. People will deal. And if they don't, you can find new people, there's a lot of those out there.
IMO life is all about compromise. You will never achieve all your goals and dreams without giving up something else as well.. Even if you win the lottery and strike it rich, you can't have it all. Figure out your priorities. If you want kids, and that's an important part of your future as you see it, figure out how to make that happen. Figure out what to sacrifice to make it happen.
Life is about ups and downs. Some people seem to strive for a perfect state of being, but that doesn't exist. Even if everything aligns perfectly for you and you are living an amazing life that you always wanted to, you will experience good days and bad days. Take the bad days as they come, you won't get rid of them. Don't be afraid to fail. Learn from your failures.
Cut toxic people out of your life and fill it with likeminded positivity instead. It's not easy meeting new people when you're an adult, but it is possible. If you only have 2 friends and they're big jerks, it's probably time to move on. If you walk into a room and you are the nicest or smartest person there.. you need to find a new room. Surround yourself with people you respect.
In the end living your best life is something only you can figure out.. and IMO most of us don't really understand what the needs of the person who lives inside us are. Spend time trying to figure that out. Going on a solo 2 hour long walk through the city or park is not a waste of time - it can be a perfect time to reflect, as you walk in a steady rhythm - or sit back on a bench overlooking some picturesque spot.
I'm not sure what to say about depression, but it's hit me in the past too. It's tough to say how hard it hit me, so I can't really say I can relate. Positive friends will help a lot though. If you're constantly surrounded by toxic people instead, that will probably not help your depression at all.