Yellow mucus is a sign that your body is fighting off something. The mucus typically darkens over a few days time. Then as the virus goes away, the mucus begins to get clear again.
If it doesn't get better or if it gets worse after a week or two, it may be an infection. Then you would want to see a doctor.
Otherwise, drink plenty of fluids like water and juice. Get lots of rest. If you need some medicine to help with the symptoms, ask your pharmacist what type of cold medicine he or she would recommend.