I’m also only a senior:D
First of all, is it a Catholic retreat or a general Christian one? That makes a big difference!
Perhaps you can begin by naming off, and maybe giving a short story about real difficulties in our society, and how hopeless and confused people are. And then move on to point out that the only One to turn to is God. You could go on to quote on how much He loves us, and that we must reciprocate this love. Use quotes from the bible and catechism (like in my signature!) to show this. And then move on to talk about different prayers and their origins. Two really powerful prayer histories are the Rosary (revealed by Mary herself, Divinely inspired), and the St. Michael prayer (Pope Leo the _th recieved a vision from God about how the devil gained permission to wreak havoc during one century… 20th!! and then he composed this prayer to help us in that century… and its aftermath) Possibly also talk about the Real Prescence in the Eucharist and how many times the host has turned into real human flesh, from the heart… and how the blood and genetics match up across the centuries and the continents! Then also talk about Fatima if you can, with the Miracle of the Sun. And how people who live the most spiritually rich lives and become saints, are often found to have incorruptible bodies after death. And you tie it in that when we love God back, and show this in our lives, that we can escape the depression and evil of society and become one with Him, living our lives in full submission to His will, and receiving eternal happiness. That we can’t justify our spiritual inactivity in any way… God is here, God is listening, God loves you more than you will ever know. By being lazy, we give up eternal happiness in place of eternal torture and agony.
Well… just some ideas;) Keep us posted!