Thanks for consulting H.C.M.
Your query is very clearly telling us that you are having upper respiratory tract
infection in the form of sore throat
and post nasal drip
probably due to involvement of sinuses giving rise to headache as well.
You may do-
1- X-Ray paranasal sinuses
2- Culture and sensitivity of throat swab.
3- Blood- CBC, ESR, blood sugar ( F & PP )
By this much and a thorough examination your diagnosis and line of treatment may be decided. Till you are waiting for investigations you may take-
b- Tab Paracetamol
d- Warm saline gargling & steam inhalation.
e- Adequate fluid orally.
I think above line of action is answering your query.