Gst compliance in startup

Hello everyone 

I am an engineering student with a startup idea. After some research online, me and my colleagues come to a conclusion to start an LLP. Its basically an online service platform to provide rooms.

I have some understanding of the annual return LLP Form 11 LLP form 8 along with the turnover and capital limits and a basic understanding of GST.

I just want to understand where and when to pay GST in normal business workings ,for example If a seller is not registered ,how do i get a credit? what all things we should do to attract less tax.

Thanks