To install new theme to magento, just copy your folder to the same location as your current theme folder is available. Let me give you an example.
Your Theme Folder path:
1. App->design->frontend->[yourtheme]->[yourtheme]->layout and Template Folder
2. Skin->frontend->[yourtheme]->[yourtheme]->Css and image folder
Now just copy your folder and paste to that place.
1. Magento->app->design->frontend->[Paste your theme folder which contain layout and Template]
2. Magento->Skin->frontend->[Paste your theme folder which contain Css and Image]
3. Magento->JS->yourtheme Js folder.
4. Magento->media->yourtheme folder.
Now apply the theme for admin interface at the following option.
System Menu->design Option->Add Design changes->select your new theme.
Hope this will help you.