I am 20 yr old & a working girl.My weight is just 40kg & my height is 152cm . I am not able to put on weight. I eat quite fine, never miss my meal, but still not able to gain weight. Acc. To the doctor I am fine from inside(no health issues). Please help with it. How should I gain weight?

The key is to put on the right kind of weight. You don't exactly just want to put on a bunch of fat, that's ugly and unhealthy.
You want to put on some muscle (as well as fat if your body/fat ratio is low). Muscle weighs more than fat, and looks nicer, and is healthier.

To do that, add a little strength training to your days, and get plenty of protein, fiber, and enough carbs and calories to be slightly over your necessary daily intake.

You'd have to find out how many calories your body needs each day. Suppose it is about 1200 calories, then you want to strength train (not just exercise, you don't want to burn fat!) and get closer to 1400 or 1500 calories. The protein shakes will help build the muscle fast, and the extra calories will slowly store some fat away.

Don't try to put on weight by shoving down ice cream and candy bars! That messes with your blood sugar levels, metabolism, as well as your energy levels.

You should get a quick checkup, have them measure your official weight, as well as your bodyfat percentage. Normal percentages for woman can be in the 20s. If your bodyfat is 15% or less, then you need to put on some fat as well as muscle, so increase the calories and carbs a little as you continue to get protein after your workouts.

This will make you feel a lot better, be stronger, healthier, and more fit-looking then just putting on fat pounds with sugar meals.
The other thing I'll warn is not to try to gain weight too fast. Don't just triple your carb and calorie intake. You want to gain slow, perhaps just a couple pounds a week. You want your body to always think things are going "normal", and not severely overloaded in any direction, whether losing weight or gaining.

So visit the health food shop, pick up some powder protein weight gainer, and mix your own shakes unless you can afford to buy shakes, or protein bars for that matter.

In the end, you should put on some muscle pounds and look and feel great, and put on some fat pounds to cover them bones up. The work outs will make you feel so much better, the protein and proper nutrition will do its work nicely. Just eating candy and ice cream will make you feel miserable and sick and you'll have no energy. Then you'll just put on fat pounds and look flabby even if you are a smaller person.

Women are often scared of muscle because they don't want to look like buff little guys or something, I don't know. But don't be scared of it. Muscle has a defined shape, it wraps around you in nice curves and is not flabby or mushy. And getting super ripped and buff looking is actually HARD to do, professional weight-lifting women have to work hard to look that ripped, it's not going to happen to you accidentally, if you were so lucky!

And that is a start! We can talk more about specifics if you like, but that is my suggestion as a general overview.

I want to put on fat?? How to go about it? N I don't have time to exercise also

Exercise is meant to burn fat, which you probably don't want to do right now. Strength training is lifting weights, to put on muscle, which you DO want to do.

To put on fat at an easy pace, maybe a couple pounds a week, then take in a few hundred more calories than what you need on a daily basis. Not too much, not too little.
There are many sugar carbs in fruit, so you could add 100% fruit juice into your diet, perhaps a couple glasses a day. Eat some Italian meals, pastas and things like that are pretty high carbs.
If you start a weight training routine, add a protein shake after the workout. It doesn't have to be a full body workout, you can do just 30 minutes of upper body on one day, then do lower body on another day. All you need is a few dumbells, not a gym membership or anything fancy.

I once was severely underweight, when I was 11 or 12 I weighted somewhere around 70 pounds. I ate a lot of protien- protein shakes and all that. I didn't do it for very long though. Now at 18 years old I'm 5' 1/2'' and less than 110 lbs. I'm small in general, but not underweight. That's just what I did when I was underweight.