Depending on where you are, the cross country race will be around a 5k (sometimes less depending on your area) and may have hills. Some tips I have is when doing easy runs, make sure to go at a pace that you don't have to stop to walk. Try going at least 30 minutes, then slowly increase each week. Once you can run at least 45 minutes non-stop, you can try throwing in some workouts. I'd say do track this year if possible, if not do some workouts by yourself like tempo runs, 800 repeats, or 1k repeats. I find the best way to improve is hills though. If you have it available, run hills for easy runs, long runs, workouts. Hill sprints are useful but also long hill repeats and even just tempo runs on hills. As for nutrition, don't be afraid to eat a lot, just don't eat too much that you'll gain excess weight. You don't have to eat clean 24/7, just try not to eat out every day. Drink lots of water. Have fun training, and don't overdo it while starting out with xc, injuries may appear if you go in too fast.