If you have exams first thing is to study hard. As Allah says in the Quran

“I give humans according to their efforts” 

After you have studied hard do the following Inshallah you will get great marks in your exams. This is a tried and tested wazifa. It works each time.

After every salah do a sajda and in the sajda read Surah fatiha once only. 

In the sajda when you come to “iiya ka na budu va iiya ka nasta een”. Repeat it 21 times. then complete the Surah till the end and raise your head from the sajda. Then do a dua for your exam. You must do this after every salah. 

Darood/salavat 1 times before and after the dua is must.

This wazifa is very small and takes 2-3 minutes and is very powerful. Continue until your result comes out. 
Make sure you also read darood/salavat fadal from the website every friday too.