I would advise you to look over the documentation regarding development and check out tutorials on the internet, like the Alfresco Developer series by Jeff Potts‌. Access to the database is never done directly - you always use services (which you learn about in the documentation / tutorials) to interact with data.
Help with what? Did you work through the tutorial I linked? What specifically do you need to access with regards to data / structure? What did you try already? What are the issues you are having?
