PSU exams like any other competitive exams cover GK, Quant, Verbal, Reasoning and Professinal aspects...
For professional aspects what ever you have studied in commerce (+1 to CA) would be more than enough provided you have grip on basics.
However for Quant, verbal and Reasoning you may to prepare bit extra...for Quant solve books (like that of R.S.Aggarwal)...for verbal develop habbit of reading (anything - good newpaper, novels, magazines) stress on your vocabulary...
However for GK you will have to make it a part of life, u cant just grab it from books...again newpapers, magazines, net etc are good source of updating yourself (besides these also go through NCERT books, pearson book, manorma etc.) do purchase book published by Ministry of Information/broadcasting...its a good reading source (for competitive exams as well as for general purpose also)
If possible and if you seriously want job only with PSU then u can also consider a short coaching course from good institute designed for competitive exams.