Re: Help with eating right
If you're running cross country I doubt you can really put on too much weight, but I could be wrong. You certainly don't need to worry about dieting.
Don't do crossfit in addition to your daily practice. Your body needs to rest when you're not running, otherwise you'll be crossfitting to the detriment of your races. If you want to pursue crossfit and weight gain, do it when you're not in cross country season.
Many people will tell you that it's very difficult to build lean muscle without gaining fat, and they're right. Since you're young, and your metabolism is likely to be enormously fast, I wouldn't suggest that you adhere to the strict Paleo or Zone diets found here. Each is great as far as healthy diet practices go, but they can be a pain if your only goal is adding muscle. It can be done, but the short answer is the same as it would be anywhere else: eat 2-2.5x the amount you currently eat.
That said, as long as you're in cross country it may be your destiny to be underweight. I did not know a single long distance runner that was muscular. They were all skinny.
If you really want to stick to Paleo, that's fine. What matters most in your case is upping your calorie intake. If you do hit the food and gym after your season, both will be a strain on your body, and at 15 I doubt that you have much experience in the weight room, so I do NOT think you should just dive right in and start tossing weights around. Take your time, slowly increasing the amount you eat, and learning proper form with a friend or coach in a weight room. Anyone that has spent time in a gym knows "the guy" that is just starting out, using bad form, and hurts himself by getting overconfident. Don't be that guy.
Once your form is good and you're familiar with the exercises, up the amount of food you eat and lift hard. Above all else, be careful. I can't stress that enough. You may be young, but you're also underweight. Your body will need to adjust to the weight lifting.
GOMAD (Gallon of Milk a Day), in addition to lots of food, is also an option for somebody starting out, but at your age and weight I'd do half a gallon at most. People commonly put on 20lbs in a month doing GOMAD, and I fear that would be too much for your body to handle.