It seems that this is a popular question because someone wrote a whole book about it:
Includes helpful questions for an examination of conscience based on the Ten Commandments and prayers of sorrow in preparation for Confession.
A helpful tool both for people who regularly go to Confession and for those who don’t frequent the sacrament. Useful, too, for pastors and spiritual directors who often face many of the questions treated in this book.
The perfect size for slipping into a purse or coat pocket, Why Go to Confession? is an enlightening read for anyone struggling with the sacrament of Reconciliation.
For something a little longer but something you can start on tonight, I recommend Pardon and Peace which is available in both Kindle and Epub formats for immediate download.
But I can go straight to God!
If you aren't really sure about why confession exists, let alone why you need it in your life, I recommend reading Lord, Have Mercy by Scott Hahn. Being an ex-Protestant, Mr. Hahn brings a unique perspective to the sacrament. You can also get a DVD or CD that contains much of the same material.
Like I said, confession is for everyone. We have several books for kids including My Confession Book which is always a bestseller as well as Going to Confession. The low price makes these perfect for a class.
If you are looking for a memento for someone receiving First Confession, we have the unique Today I Made My First Reconciliation book which explains what confession is and also has a place to record the event.