To authenticate to the api you need to create a "system" user. Head to your channel's Settings > API Access and create a system user choosing scope. The user's name it's just a description for your convenience, it doens't matter.
Scopes:
- Admin: You can do anything
- Schema: You can manage content and schemas
- Content: You can manage only content
If you only need read access of your content, then better use the Content API.
To connect to the API use the following headers
Lucent-Channel: {channel-id}
Lucent-User: {sys-user-key}
Lucent-Scope: {live or demo} # default is demo
Authentication: Bearer {sys-user-secret}
Accept-Language: {a-channel-locale} # this is optional