You need to login to your servers control panel and install the missing extension. I do this fairly painlessly via Easy Apache on a Centos VPS running WHM.
The process will differ depending on what kind of hosting you have and wether it’s powered by WHM/cPanel or Plesk or something else. In all cases though it should be possible to do it manually via the command line, although this can be a little scary. If you have shared hosting (which I doubt, that extension should be there already) you probably won’t be able to install it because it will affect all sites on that server and hosts generally lock you out of doing so.
It’s also possible you already have that extension installed but it has been disabled in your servers config files. You will need to edit those to enable it, if that’s the case.
If you give a little more info about what your server is running, we can point you in the right direction, or alternatively you can talk to your web host and they may do it for you.
You said in your first post that other pages apart from the start page are. not available either? So it seems that the .htaccess file is not working as expected. Have. you made any changes to that file?