If you are using self hosted wp blog then you need to upload all plugin file including folder under plugin directory and then you need to activate that plugin through your wp admin. DON.
or if your using an up to date version plugins can be uploaded/installed directly from the backend admin menu wp > admin > plugins > add new
Install plugin in wordpress is very simple.Extract the plugin zip folder and upload to wordpress plugin dir.after taht go to admin section of wordpress and than go to plugin menu option and than activated the your new plugin.i hope this will realy help you to install new plugin..............if you are still not able to install plugin please PM
Another simple way is to install directly online!go to admin section of wordpress and than go to plugin menu option and than click add new ,search for the plugin u want to install and click install !
if you are using wordpress blog script on your hosting domain then only you can install plugin other wise if you have created a free blog then you cant..install plugins