I never masturbated in my life because I'm not able to get aroused and attain any erection

You must coornsult a doc.

Hi, it all most sounds like it could be more physiological reason why your unable to get an erection, do you even get semi erect or can't you even manage that, was it just that you didn't know about masturbation or was your brought up to look upon sex as something that was wrong?

Wet dreams are common between the ages of 13 to 16 when the testosterone levels are elevated during puberty and it's normal for them to drop off once into your 20s. 

Without knowing your full history it's hard to say what else it could be. 

Hi, thanks for replying. Yes, I am able to get semi erect but not always. Also, when I get some erection I am not able to hold it for long.

No, It's not like that I have been brought up in a conservative surrounding, where sex is looked down upon as wrong thing. Its was just that I didn't knew about the jerking act. I used to play with it, get it erect and enjoyed getting a boner but I didn't knew about masturbation.

Please suggest me, How can I know what the actual problem is, what can I try to improve my condition.

I take it you haven't watched a lot of porn when you was younger if you didn't know about masturbation as that can cause similar thing to happen.

Well this is not the case, I have enough porn movies since childhood, Its just that got to know it a little late that a hand can also be used as an alternative, you know what i mean.
I do not understand if it is a Hormonal Deficiency issue or some other problem, but I have undergone S. Testosterone levels lab test, which I found to be in normal range(400 ng/dl).
What could be the problem, and how can it be diagnosed ?
Aren't there any tablets to enhance sexual stimulation, which i can try ?

I am much worried about now about my sexual life, Soon I am going to get married, and if am not to do it, it will turn into a disaster.

If you was able to get and maintain a hard erection when you was at a younger age then it won't be a hormone problem or you wouldn't have got them then.
My theory is that it is more down to psychological issues than anything else, you could take viagra to help you get a stronger erection but you need to get to the root of the problem on why your suffering erectile dysfunction because once you start taking them your find that a normal spontaneous erection will be hard to achieve with out taking pills.
Try googling erectile dysfunction and see if you can find any help on it there because the only person that can rectify this is you if a doctor says they can't find any problems.

Thoughtful mothers, when they begin to notice frequent erections in their boys, will begin a special nighttime ritual at least several times a week. At bedtime, the mom will give her son a very relaxing and thorough back rub which puts the boy in a very passive relaxed sleep, with many boys almost asleep at that point. The mom gently helps flip the boy into his back and proceeds to give a very soft and soothing massage to the boys neck and chest. This puts most boys into a quasi-sleep mode. At that point, the mom simply moves the massage lower on the boy’s body, including his boxer briefs. Many boys at this point aren’t mentally aware or register what’s going on. He won’t notice (or care) when her fingertips start to gently massage his boxer briefs. Won’t notice when his penis begins to lengthen and stiffen inside the briefs. But mom will. She will then begin to focus her massage on the material over the head of the boy’s penis, the most sensitive area of his young body. He will attain a full erection very rapidly. He won’t notice when her fingers then begin to massage the glans of his penis, below the head, nor that her other hand is gently massaging the fabric covering his testicles. At this point he is fully sexually aroused. The mom will soon feel the wetness through the fabric of his precum. At this point, the mom will stop and use her fingers to open the slit of the boxer briefs, allowing her son’s erection to naturally come out in the open air. Using her son’s own precum as a lubricant, she can gently massage his entire penis with her hand to bring him to a full and quick ejaculation in his sleep. Some mothers instead will simply lower their mouth onto the head of the penis and using a combination of sucking and licking the head, bring the boy to ejaculate directly into her mouth.