From what I've seen as a student nurse at an oncology ward (mind you I'm not an oncology nurse) is that young, previously active people fare best on staying active. Keep moving. Even if it's just sitting up in a chair for an hour or so if that's the most you can do or doing some bike pedaling. Do not let yourself be bed-bound unless you really are too sick and fatigued to even get up (this doesn't happen a lot to young people). Realise that for many people the real hit only comes after two or three weeks.

Keep your friends and family close. If you can't physically make sure you Skype, facetime, whatever. Social contact is a huge energy booster. But make sure you keep the friend of a friend of a neighbor type visitors at bay if you are fatigued. Save that energy for the people closest to you.

Last thing, do not be afraid to say you are in pain. Radiation can take a lot of pain away but it can also give you terrible nerve pains. Pain pills aren't the devil and they'll really help you. Pain is one of the most hindering symptoms but also one of the easiest to solve usually.