Objects
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.
Setting up an Object
The minimum steps required to set up an Object.
Create the Object, name it
Set the Name field on the object - eg Auto Number
Set the details on the Object
Allow Reports
Allow Activities (optional)
Track Field History
Set Deployed
Allow Search
Create Tab
Create the Tab for this Object
Decide who should access the Tab
Decide which App the Tab should be in - HINT! NOT ALL OF THEM!
Optional - Create Record Types
Add the Fields to the Object
Include Help Text
Set correct Permissions as you are creating the field
Include Lookup or MD fields to other objects.
Remember to set the Lookup Filters if required.
Edit the Page Layout to ensure that all the fields are visible and in the right location
Make the Section header visible for System Fields
Ensure required fields are required.
Add Files and Notes related list (if you are using them).
Add Object History related list
Override the Salesforce Mobile and Lightning Experience Actions
Set the exact Actions you want to appear on the Object
Optional - Create other Page Layouts if required for other Record Types
Optional - Create a Create Record Action from a related Object
Ensure the Layout is correct with all the required fields required for Creating the record
Optional - Create an Update Record Action for this object so records can be bulk updated from the list view
Create the Compact Layout and assign it to the Page Layout
Set the Search Layouts - IMPORTANT!
If there are any buttons to add to the List View, then add them
Optional - add any Validation Rules
Set permissions on Object
Optional - Set Record Types defaults
Enable Feed Tracking for the Object
Look at the Object, create a Test Record.
Edit the Lightning Page. See Lightning Record Pages
Chatter first
Delete Related
Add Related List Quick Links IF there are related lists.
Add Files as a Related List under Chatter.
Activate Page - Assign as Org Default.
Go to the Tab
Edit the All View
If there are any Record Types, create a View for each Record Type
(Ensure you go back to Classic if this will be in Communities to ensure Communities users can't see Inappropriate Views).
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.