SSL on several pages how to do and which plugin

Just got the SSL activation for my site. On my site there are only a couple of page with forms I want to protect. Those pages are: contact – registration – customer upload media + documents – whole admin.

But how do I install SSL for those page and which plugin is suitable for this?
Or should I protect the whole site with SSL and when I do do visitors get problems viewing the site? This is already active.

Maybe someone can inform me what best solutions are and why.
Till now I am a newbie.

Regards,
Norus