Trailhead provides a really nice overview of Salesforce Custom and Standard Objects and how they are structured https://developer.salesforce.com/trailhead/force_com_introduction/data_modeling/objects_intro.
Fields
This video from Trailhead is a really good overview of creating fields https://www.youtube.com/watch?v=j95gONjVNFU#t=11
- The most annoying thing about fields is not seeing ALL fields in alpha order - so have a good idea of what are the custom and standard fields in your org.
- Also see each object below for the standard fields that are not set as visible by default, which is kinda annoying (eg Contacts).
- The most important thing about fields is the security https://help.salesforce.com/HTViewHelpDoc?id=admin_fls.htm
Relationships
Trailhead has a good overview of Relationships https://developer.salesforce.com/trailhead/force_com_introduction/data_modeling/object_relationships
Remember, Master Detail relationships are great, but you can always use triggers, or Rollup Helper to create calculated fields if you don't have a Master Detail relationship.