Is there any precaution you need to take while using ubuntu? Because i dont use any antivirus available for ubuntu! So any security changes do i need to make while surfing on the net?
Ubuntu is inherently more secure than Windows because of its Unix heritage, which was built for security from day one. Also there are apps in ubuntu like app armor that provide protection against buffer overrun attacks in your web browser and other programs.
The concept of root users is something that has come from Unix and makes it impossible for a user to do anything that could permanently effect your machine without being a root user or being temporarily promoted to root (such as with sudo). Because root is so powerful be very, very careful what you allow to run as root because once it gets you permission to run as root it can do anything it wants.
Other than that really the only thing you need to do (if you are running a desktop machine with no servers running on it) is keep the system up to date.
Good to see you are concerned about security. i assume you are a windows user, who has switched to, or trying out ubuntu. You do not need an antivirus product, because simply said, the architecture of linux is such, it is much less prone to malware. If you are still concerned you can use solutions like clamav and gufw ( available in software center) tpo further strengthen your defenses.
Use script-blocking extensions for your browser that are available for Firefox, Chromium and Midori.
The best of these script-blockers is easily No-Script for Firefox which is capable of not only blocking scripts, but also of providing Cross-Site-Scripting protection for those websites which you have whitelisted. It also blocks plugins such as Flash and Java, click-jacking and web-bugs. At first No-Script can be annoying because it will cripple many websites you visit, but gradually, as you whitelist the websites you trust, it will bother you less and less. I recommend to learn the keyboard shortcut to bring up the contextual menu (which can be changed in about:config) and to read a little the documentation on he web-site.
An Ad-blocker is something else to consider, since many of your trusted websites may serve you dodgy ads.