I am experiencing post nasal drip, cough (quality: with phelgm or mucus) and nasal congestion. The following also describe me: Runny nose, Sore throat, Sneezing, and Nose itch. What should I do?

Try nasal steroid. These are suggestive of an allergic problem. You may try topical nasal steroid for a week. But if there is no relief , consult an allergist.
Doctor can examine. New throat pain can be from a strep throat infection (antibiotics needed), a cold or influenza (antibiotics usually not used), or from some other cause such as allergies or irritants. Usually, a person sees the doctor if the sore throat is bothersome, recurrent, or doesn't go away quickly (mainly to look for strep). Flu season usually runs from Thanksgiving (end of November) through February.