Decide what you want. Career is a big factor, as you’ll spend so much of your waking life engaged in it, but consider things like location, financial status and family as well. Do what you love, or what you think you could love. If you don’t know what you want, build toward it in a general way: take some general classes in college, especially if your lifestyle is likely to require significant education, socialize and travel a little, make some money. Dabble in a few things that interest you.
When you’ve got a better idea where you think you’d like to be a few years down the road, build toward it in small steps, as Foxboro said. Get an entry job in the field, take specialized courses if you’ll need them - and along the way, try to enjoy yourself. As Melt said, even if you do attain a relatively ideal life, you’ll still want something. If that’s all you’re focused on, those things you don’t yet have, you’ll be miserable. So no matter where you are in life, set some time aside and do what makes you happy. It’s an exercise in balance, but you get better at it with time.