Should i file cst return?

Dear Friends,

I'm an engineering graduate, and i'm working in a company where from a couple of months now i have to file VAT & CST returns. I have the basic knowledge and all our purchases are from only one company from Chennai, and sell only within our state. We haven't bought anything till April, so i just went on to Andhra Pradesh Commercial Tax Site and filed Output Vat Return, and filed Nill Vat return, till here i didn't had any problem.

But in May we made some purchase from the company that is in Chennai, and today i filed Vat return usually, and then tried to file CST return by going on the site.

I tried to enter the Net Purchase value in 2% column of Tax rate wise turnovers of interstae sales and tax liability. As there is no seperate column for purchase and sale as it wasd in Vat return, and i got a meassage that it should match with TURNOVER SALES or something.

Well so i checked last year's returns, and our accountant entered the purchases as Excempted in Vat Purchase form and continues with sales return, and filed a NIL CST return. And we issued C-Forms for last year's invoices this year.

I clearly don't understand the concepts, and i know this post is very long, but Please do Kindly try to help me.

Thanks.