Backup shell script, how do I write one?

I have to write a shell script that makes a nightly backup of three files and I don't have a clue in hell what I am doing....any sites that would have an example of one similar, so I can understand this crap?! I am so frustrated and upset because I have a hard time understanding the lingo for this god awful cl and this ignment is killing me! Please help. I have been googling this stupid ignment for 2 weeks with no help.....