You can customize each of the standard tabs and types of records, including adding custom fields and setting page layouts. You can also customize search, tagging, and user interface options for your organization.
Navigate to the Setup
- Click on Your Name
- Go to Setup
To customize: Accounts, Contacts, OR Activites in the system
Navigate to Setup | Customize. Then click the standard object you wish to customize. For example, Accounts, Contacts, Activities, etc.
Accounts
Navigate to Setup | Customize | Accounts. Customize the standard picklists, create custom fields and custom links, control the layout of fields, related lists, and custom links, and for Enterprise, Unlimited, and Developer Edition organizations, create record types and set up account teams.
Contacts
Navigate to Setup | Customize | Contacts. Customize the standard picklists, create custom fields and custom links, control the layout of fields, related lists, and custom links, and create record types for Enterprise, Unlimited, and Developer Edition organizations.
Activities: Tasks | Events | Calendars
Navigate to Setup | Customize | Activities or Events or Tasks
Customize various options for tasks and events such as fields, page layouts, record types, validation rules, search layouts, and buttons. Also, enable or disable functionality like activity reminder popups and recurring events.
See Force.com's Documentation: Customizing Activity Settings