Hi,
You can download Themes directly to your blog by using the Add New Themes option in the Appearance sub-menu.
Log in to the WordPress Administration Panels.
Select the Appearance panel, then Themes.
Select Add New.
Either use the Search or Filter options to locate a Theme you would like to use.
Click on the Preview link to preview the Theme or the Install Now link to upload the Theme to your blog,
Or use the Upload link in the top links row to upload a zipped copy of a Theme that you have previously downloaded to your machine.
Plugins are utilities which provide additional functionality to your application. To install a plugin you just need to put the plugin files into the wp-content/plugins directory. Once a plugin is installed, you may activate it or deactivate it from the Plugins menu in your WP administration.
OR Follow the steps given below:
In WordPress 2.7 and above you can install plugins directly from the admin area. Download the plugin to your computer, then log in to your WP admin area and go to Plugins > Add New.
Browse to the plugin archive and select it. Then click Install Now and the plugin will be installed shortly.
The removal is also straight-forward in most cases. Simply remove the directory for the plugin you would like to uninstall and it will be automatically deactivated.
Quick Review:
To add a WordPress Plugin using the built-in plugin installer:
Go to Plugins > Add New.
Find the WordPress Plugin you wish to install.
Click Install Now to install the WordPress Plugin.
The resulting installation screen will list the installation as successful or note any problems during the install.
If successful, click Activate Plugin to activate it or Return to Plugin Installer for further actions.
Thank You.