Assuming you are new to VMware certifications such as VCP, there are 3 mandatory requirements:
- Attend authorised training
- Pass the vSphere Foundations exam
- Pass the VCP exam
The actual training and VCP exams vary from VCP to VCP, but the principle is the same for all VCPs.
Today, for the DCV specialisation, there are 2 separate VCP tracks - you can review the specific requirements of each on these pages:
It usually makes sense to take both exams on the same version - so either 6.0 Foundations with the VCP6-DCV exam, or 6.5 Foundations with the VCP6.5-DCV exam - note that the VCP6.5-DCV requires that you take your training on vSphere 6.5 though.
You can take meet the 3 mandatory requirements in any order you choose, but until you've completed all 3 you will not be certified.