When doing a search for SEO plugins there are 34 pages of possibilities. Some claim to help with the SEO of your entire WordPress site, while other plugins claim to boost the search engine effectiveness of a single function. When in doubt, find out what others are saying and recommend.
It doesn’t take long to see that WordPress SEO Plugin by Yoast is what you are looking for. It comes highly recommended. It is stable (meaning it isn't something new that is being tested out) and it is updated regularly. When you combine that with good tutorials on how to use it and good support, you can quickly see why this plugin will take care of your WordPress SEO needs.
Check out SEO by Yoast