If you want to Create new theme for your magento or you don't want to make any changes in your base theme than follow this easy steps:

• At the first stage we create folders for our new theme. We call it mytheme. Then we add a new folder with the name of our new theme.
• It is important to remember! Before changing .Phtml file, it must be copied from base (or default) themes to a new theme. When Magento detects one of the files in a new theme, it will ignore .phtml file from a base (or default) theme. Copy only those files that will be changed. It minimizes the number of errors during Magento upgrade.

Step1: Go to app/design/frontend/base/default and copy that all folders

Step2: Now go to app/design/frontend/default/mytheme and paste here. Make sure there is already default folder. You don't need to change anything in that default folder. 

Step3: Now go to skin/frontend/default/default/ and copy that all files and folders and paste it to skin/frontend/default/mytheme.

Step:4 Now go to your admin system >> configuration >> Design and change your new theme name here.

Change mytheme name with whatever your theme name you want make it.