Well, I can't choose what YOU want to do in life. Choose something you are passionate about. Do you like to draw? You could select your biggest goal in life as become a famous artist. Do you like to cook? Maybe you could select your biggest goal in life as being featured on Chopped. Although I can't help you with exactly what to write, here are some tips:
-Don't fake it! If you want to be a famous video gamer then go ahead and write that!
-Feel free to make something up if you aren't sure what you want to do in life.
-If you have a something you enjoy but think is not worthy for being something that you can set as a goal in life, you are wrong! In this point in time, everything can be turned into a career or dream of some sort! Even something as simple as reading! Maybe you want to be an audio book reader! Or a writer!
-If you are having trouble figuring out how to get 78 words try to fill those spaces in with "unnecessary" information. Maybe add a backstory as to why this is your biggest goal? You can add some motivational things family and friends have told you that pertain to your hobby, dream, etc.
If you could maybe tell me what a hobby or liking of yours is I could think of some goal ideas for you if that is what you are having trouble with so feel free to reply to this, I'll try to help!