Presuming the appropriate security groups have been applied to an instance, you can connect via remote management protocols such as SSH (22/tcp) or RDP (3389/tcp). The authentication credentials required to log into systems remotely will be based on how you have built your images.
Metapod reference images follow a standard authentication model. These Linux images allow remote login via SSH key pair to the cloud user. Password-based remote login is disabled for all users by default. Thus, an SSH key must be configured and specified at the time of instance launch for remote login to function.
Once you have logged in as the cloud user, you can sudo to root without a password. These images should be locked down further, per the guidelines and best practices of your organization, as part of preparing the systems for their intended workload.