Hello, I will be applying to colleges this fall. I had a general question about applying to colleges and getting a wait-list letter. What should I do if I receive such a letter, and what is some good advice to follow in order to show the university that I am a serious and dedicated student who would love to attend?
Well if you have an interest in the waitlisted school, then you should accept the offer. If you are content with your current decision then you should turn it down, giving those who actually care a better (but still extemely small) chance of admission. You can take initiative at the schools in which you are interested by keeping them updated with a letter or two, just so they know you care about getting accepted.
What Ivystriver2 said is generally good advice, but seriously: don’t worry about it until it happens! You haven’t even applied yet 
Focus on making your apps as good as they can be right now, and maybe you won’t even need to worry about waitlist letters.