Google Calendar is a free, online and convenient service. Desktop calendar applications are great if you're always at your desk, but if you like to be more mobile, you should consider switching to a calendar that you can use from anywhere you have internet access. So, if you need a calendar that is free, online, easy to us, and has lots of fancy features, this may be the one for you.

How to get started

Google Calendar Standard Features

• Create multiple calendars: You can create multiple calendars for different aspects of your life (work, family, etc).
• Sending invitations: You can invite people to any event by simply adding their email address to the guest section of the event. Google Calendar will send the emails.
• Event reminders: Option to get notified by email or text messages sent to your mobile phone.
• Works closely with Gmail: When you receive an email about an event, you can add it to Google Calendar with just a few clicks.
• Calendar Sharing: Your calendar can be shared with others, and you can subscribe to others’ shared calendars.
• Calendar Search: There is a very helpful search facility.
• View modes: Calendars can be viewed by day, week, or month.
• Show or Hide Events: Each calendar can be set to either hide or show specific events.
• Monitor/Manage calendar from your desktop using google sidebar.
• SMS Events: You can update your calendar using an SMS from your mobile. Check out the Calendar>Settings>Notifications option  for more info on this.
• Emailed Agenda: You can have your full daily agenda emailed to you from your calendar. Check out the Calendar>Settings> Notifications option for more info on this.

Read this link for more details on how to use Google Calendar.