Recently I have installed ubuntu12.10 and got internet connected. I w'd prefer to work on ubutu only and not to go back to Windows. Please suggest me how to keep my ubuntu system maintained? For that, please advice which antivirus should I purchase/get? May I get one from ubuntu / UNIX website?
Also help me with commands to get the antivirus. I really prefer and enjoy doing things via command line riather than GUI like Windows.

Keep your packages updated (sudo apt-get update / sudo apt-get upgrade), same applies for the plugins of your browser (Firefox: Add On Manager, you find a link there).
Actually, there is no real need for an antivirus program on Linux, since most of the malicious software is written for Windows and cannot harm a Linux system. If you execute it with Wine, it can destroy your "Fake Windows" only.
The few pieces of malware for Linux are AFAIK all requiring root rights to do real damage, so if you always obey rule No. 1 (Administrate as root, work as user), they aren't harmful.

there is some useful information here, but as suggested therein, most people do not run any AV at all under Linux. https://help.ubuntu.com/community/Antivirus

Thanks thelinuxist and acid_kewpie for the useful information. It's surprise for me though to learn this.

The ubuntuforums.org site has an excellent sticky post in their security section. Normally, I would avoid suggesting a different forum, but this particular post is written specifically for someone in your situation asking these very questions. It discusses the difference between Linux security, with a focus on Ubuntu, and Windows security and explains how you need to approach things with a different mindset. For example, in Linux, you don't really need an "antivirus" program, but you do need to understand the file system and permissions, largely for the same reasons Windows needs an antivirus system.

Once you have absorbed the information in that post, the security references sticky in this forum is an excellent resource for more detailed and specific information.

Noway2, thanks a lot for your valuable help. The links seem to be wonderful.