The people reading your essays are regular human beings, which means you should write with that in mind. A good way to check your tone is to read your essays out loud. No, not in your head, out loud. Read them to a friend, parent, sibling, whatever, and if you feel embarrassed or uncomfortable by the style, then you should change it. This doesn’t mean you should add in colloquial filler words like like, um, and uh, but it means that the essay should flow smoothly enough that you feel comfortable reading it out loud in front of someone you don’t know very well (don’t actually do that, but you should feel good enough to).
The popular "topic of your choice" option had been removed from the Common Application between 2013 and 2016, but it's now back again for the 2017-18 admissions cycle. Use this option if you have a story to share that doesn't quite fit into any of the options above. However, the first six topics are extremely broad with a lot of flexibility, so make sure your topic really can't be identified with one of them. Also, don't equate "topic of your choice" with a license to write a comedy routine or poem (you can submit such things via the "Additional Info" option). Essays written for this prompt still need to have substance and tell your reader something about you. Cleverness is fine, but don't be clever at the expense of meaningful content.
Avoid slipping into clichés or generalities. Take this opportunity to really examine an experience that taught you something you didn't previously know about yourself, got you out of your comfort zone, or forced you to grow. Sometimes it's better to write about something that was hard for you because you learned something than it is to write about something that was easy for you because you think it sounds admirable. As with all essay questions, the most important thing is to tell a great story: how you discovered this activity, what drew you to it, and what it's shown you about yourself.