Hello,
There are many guides out there that explain how to set up projects and start up with GCP. You can see the official page of getting started with GCP. Now, while going through these guides, you may experience issues which for you would need to seek help individually. The good news is that there is a lot of information out there within the documentation of GCP, Stack overflow and many third party websites on how to proceed.
As for your immediate specific problem with the Cloud Build service account, you can take a look at this doc which explains how to add permissions in general and this is another doc that explains the permissions for the Cloud Build service account. Looking at these two documents, you can get back on track with the Cloud Build service account.