You can't develop Apex in your Salesforce production organization. Live users accessing the system while you're developing can destabilize your data or corrupt your application. Instead, you must do all your development work in either a sandbox or a Developer Edition organization.
If you aren't already a member of the developer community, go to http://developer.salesforce.com/signup and follow the instructions to sign up for a Developer Edition account. A Developer Edition account gives you access to a free Developer Edition organization. Even if you already have an Enterprise, Unlimited, or Performance Edition organization and a sandbox for creating Apex, we strongly recommend that you take advantage of the resources available in the developer community.
For a Partial Copy sandbox, click Next, and then select the template you created to specify the data for your sandbox. If you have not created a template for this Partial Copy sandbox, see Creating or Editing Sandbox Templates.
For a Full sandbox, click Next, and then decide how much data to include.
To include Template-based data for a Full sandbox, select an existing sandbox template. For more information, see Creating or Editing Sandbox Templates.
To include All data in a Full sandbox, choose whether and how much field tracking history data to include, and whether to copy Chatter data. You can copy from 0 to 180 days of history, in 30-day increments. The default value is 0 days. Chatter data includes feeds, messages, and discovery topics. Decreasing the amount of data you copy can significantly speed sandbox copy time.