To register a Namecoin name, you need a Namecoin wallet that supports name operations. Currently, that means you want Namecoin Core. Namecoin Core includes both a Qt GUI (Namecoin-Qt) and a command-line version (namecoind). You can download Namecoin Core at the Downloads page.
After you’ve installed Namecoin Core, you’ll need to obtain some NMC (the digital currency used by Namecoin).