Start at the beginning. Pick up a pen and paper.
Make a list of what you do during an average day. What do you enjoy and what do you dislike?
What do you do for fun? For stress relief?
What subjects did you enjoy in high school?
Do you like books? If you have a home library look and see what types of books you have. If you go to a library, bookstore or magazine stand what section do you visit? What do you avoid? (What you don't like is as important as what you do like in your quest to find mission).
What bookmarks do you have on your computer? What kinds of things do you search for if you are wasting time online?
Are you a physical person or an intellectual type? Do you need a daily physical workout or daily mental stimulation?
Would you rather be outdoors or inside working in office? Can you do repetitive work or do you require a changing environment? Do you need structure or freedom?
Are you an introvert or an extrovert? Would you rather work alone or in a group? Do crowds drain you or energize you?
Would you rather be the boss or the employee?
Do you like to travel? Do you need a 9-5 work schedule or flexible days and hours?
Do you do volunteer work? Do you have hobbies?
If you had to write your own obituary what would it say? What do you want to be remembered for after you die?
I strongly recommend career and personality testing. There are a few good tests, some of which are free and online. Myers-Briggs is excellent but online will cost you. You can get the book and take the test inside the book. My results were the same.
Your college will also offer career counseling. If the testing is free you could take it, although I would be cautious about career counselors themselves. They will pressure you to make a decision when you are really getting to know yourself. The answers emerge afterward.
Another thing, if you are called to marriage someday, make sure you have an idea of your mission first because in addition to spiritual concerns you want a wife who will support your mission (Eve created to be Adam's companion and helpmate).
The Four Temperamentshttp://www.fisheaters.com/quiz1.html
(the online test is here. I am not endorsing FE).http://sedevacantist.com/misc/temperaments.htmlhttp://www.keirsey.com/
(similar to Myers Briggs but free)