Yes, you can create customized fields for your Accounts, Contacts, and Opportunities.
Select the entity you want to add custom fields to by clicking on either the "Account", "Contact", or "Opportunity" tabs in the top bar.
Click on the large orange "+" button at the bottom right of the screen to create a custom field.
Select the right field type and give it a descriptive name.
Learn more on what predefined custom fields are for.