I buy from one state and sell to another state.

If you get register in GST, you will get credit of GST paid on your purchases. If your purchases are inter-state then you will pay to supplier IGST and you will get credit of that.

If you make purchases locally, you will CGST and SGST to supplier and you will get credit of this also.

Agree with sukesh sir,

In short earlier-100+2=101 was your cost.

Now 100+18=118 but 100 is your cost.