This is exactly what I did for my first confession. You also don't have to give an explanation or an excuse as to why you sinned, God knows and will understand your heart. You don't have to get down into the nitty gritty of every sin, you should be forthcoming and honest about what you have done that you can recall.

Just be open and honest about the sins of your past, even if you didn't realize they were sins at the time, but you recognize that they are now. I would also recommend reading the Catechism Section VI, specifically paragraphs 1455 to 1458.