Install SEO Plugin

Search for an SEO plugin using the Search field.

This will produce a list of relevant plugin. The most commonly used is probably the All-in-One SEO Pack. However, not all plugins are listed on the WordPress list and another good one is WordPress SEO Master from http://www.ezbizniz.com/.
Install the plugin by clicking on the Install link in the far right column (not shown).

Click on the Options configuration panel link to open the panel.
The new SEO plugin will also have appeared in the Settings menu and the configuration panel can also be reached from Settings.

The plugin default settings are fine with one exception. Part way down the page you will find these settings. DO NOT use noindex on Categories, Archives and Tags. There are many opinions on this subject. This recommendation is based on 2 points.

Firstly, page rank is based on the clustering of links and the more clusters you have within your site the better. The Archive, Category and Tag pages are all natural clusters within your site, they are understood by the search engines and add to the power of our site.

Secondly, trying toinfluence the search engines is a dangerous and losing game. You are basically trying to make their work more difficult and you are up against the biggest collection of Ph.D graduates and computing power on the planet. They know how WordPress is structured and if you try to influence their system then you flag yourself as a marketer and are at risk that they will negate your efforts and downgrade your site.

Apart from which, you have better things to do than go fiddling around with a subject you know little or nothing about. Spend your time publishing useful, relevant content in the right way and you will gain far more benefit.

Don't forget to Update Options.