How do I prevent unauthorized accession of my data when my phone is connected to the internet ? I have an Android smartphone and my data connection is always enabled. I often use browsers like Opera Mini and UC Web as they are faster than the inbuilt browser apps. I often use Mobile banking services for several purposes. I have heard that the  hackers can steal data from Android smartphones without my permission when my phone is connected to the internet.


How do I get to know that my private data is being accessed by others without my authorization?
Other than installing an Antivirus app, is there a way to prevent my smartphone from allowing access to my data? Like, is there any default inbuilt security option in Android smartphones as of such?
Yes, just like for any Linux PC, you can install a Firewall to block wifi/mobile internet accesses and XPosed to keep track of all access attempts made to you device from programs. But unlike what Google and Samsung want to make you believe. Rooting your device is essential to allow you to have far better control of leaked data and unauthorized access. I highly recommend using apps like: 


AFWall+ (The firewall to control internet access.)
XPrivacy
(Uses the
XPosed
framework for fine-grained access control.)
App Quarantine ROOT/FREEZE (Temporarily disable unruly apps.)


These are just a few fundamental steps to take to better protect your privacy, to mention a few. In addition you should also know that Google AOS/API often circumvents iptables based firewall settings on many devices. This is especially true for mobile carrier provided phones. See for example, Android betrays tethering data.

Finally, Anti Virus, is almost useless in the mobile device world, as there are so many other attack vectors that can be far more easily exploited, than what's detectable by signature based heuristics.