One big warning. A LOT of people think that JUST installing an SEO plugin is going to improve their rankings. This is not true at all. You still have to actually know SEO basics and actually use the additional fields etc that the SEO plugin has. You still also need to optimize your interlinking, backlinks etc.
So, if your theme already allows you to edit the title, description and keyword tags, you really don't need an SEO plugin.
Also, take the recommendations that an SEO plugin gives you with a grain of salt. Making the changes can and does on may occasions turn your site into a sterile wasteland that is not reader friendly and has no personality. So, you make all the changes and now, nobody wants to stay on your site and read it. So.......
BE VERY careful if you use an seo plugin. Make sure you understand it and understand SEO in the first place.