In CS exam first exam is Foundation exam in which you can appear after qualifying class 12.

Further, CS is a 3 stage programme:
1.Foundation Programme
2.Executive Programme
3.Professional Programme

CPT exam is first exam of CA exam.

In order to do CS you need to clear three stages

Foundation Programme

Executive Programme

Professional Programme

To know in detail you can refer below link

Stages to become a CS

Good Luck!!