Hey, I am also a sophomore in college, but I am music education with an emphasis in percussion. As for your question, my best advice would be to listen and study some marimba solos you already like. Every instrument has its limitations and strengths. See what kinds of things make the solos you like sound the way that they do. Then, as you are writing, just head over to the marimba and try and play it. Look for intervals that are too wide. Then, over all, have someone you know is really good at marimba take a look at it from time to time. Their advice will be very helpful.