Hi Prashant. Fungal infection of toe nail is a common in people who are in professions where there feet remain moist or have to wear shoes for long period of time. You need to keep your feet dry. Change socks twice a day. Use powder on feet before putting on socks. 

For the fungal infection already there You need a long course of around 6 months of antifungal ointment.  As u are having too much pain there may be superinfection by a bacteria. You would also need to use a topical antibiotic ointment for 7-10 days.