I am 25 yrs old standing 173cm and weighing 53kgs. I have job which is totally sitting one for at least 8 hrs a day. After that also I dont have much physical work. I want to gain weight so where should I start first. Should I start eating more and after join the gym or take the gym with more food intake. I heard that peanut butter is great to start off with. I am vegetarian so my diet will be accordingly. Thank you.
TLDR: Yes, joining a gym to work out and eat more (good) food will help you gain weight, and I'll explain how to start off.
First off, 4 years ago I was right where you are. I'm 184cm tall and would weigh about 56kg. In the first two years I gained about 20kg, but I had to work very hard for it so don't think this will come easy.
You'll need to start working out, I advice either 3 or 4 days a week. I would advice to do a mix of weight training and bodyweight training. So things like benchpress, deadlifts, squats, shoulderpress, combined with things like push-ups, pull-ups, planking. This will give you a well rounded plan which will incorporate every muscle in the body.
You'll also need to adjust your eating habits. Since you'll be burning a lot more calories because of all the working out, you'll also need to consume loads more, especially because you want to increase your weight.
Since you are a vegetarian, things like beans, lentils and all kinds of nuts will be good to increase your caloric intake. You'll also need to consume enough protein, quark-yoghurt is a good source for this for vegetarians. What worked for me is to also consume a lot of carbohydrates, so I eat about half a bread each day with things like peanut butter.
One of the most important things to note is that gaining weight won't happen overnight. You'll need to be patient with it. I hope this helps, if you have any furthuer questions feel free to ask them. Also, if I missed anything feel free to add it in the comments.
If you want to gain aesthetic weight don't bulk heavy just a 250 calories surplus, with progression.
1 year 250 surplus, the next year 300 then 350 and so on.
A good example for a diet is : 1 meal of pasta/rice 1 meal of beans/grains 3 snacks of nuts 10 snacks of fruits/veggies and maybe some fruit juice or almond milk.
If you have thyroid problems stay low on your carbs, if not then just eat normally.
Don't join any gym, it's not worth it unless you live in a really cheap country. 1 year of gym membership is 500$ or 600$ and it's only 1 single year which is not enough to make any substantial change to your body, you might gain 9-15 lbs of muscle mass and you need to pay again to keep the muscle or build more.
By buying what i listed down there you will be able to train every single skeletal muscle in your body from your toes up to your jaw. You can get a big back, huge chest, huge legs and biceps and so on for a total cost of only 269$, this is all equipment that will last forever which you can keep in your closet or your garden. Don't listen to biased powerlifters or dogmatic people, you don't need a gym for either speed,strength or muscle.
Prices are the averages you will find
a pull up bar : 25$
two adjustable dumbbels of 6lbs to 64lbs : 44$
two adjustable dumbbells of 65lbs to 200lbs : 130$
two 33 lbs bands : 7$ each
two 176 lbs bands : 25$ each
two hand grippers : 3$ each
for dips or inverted rows all you need is two chairs. or if you want to be fancy buy another pull up bar and put at chest height.
It might not seem much but with this stuff you can do about 500 exercises and each one of those can be done in different variations, and the average gym rat only does 8-9 exercises and it's all he knows.
on top of those 500 weighted exercises you can also add over 200 calisthenic moves to your options.
You can start with a simple routine. I suggest training 3 to 4 times a week and each day do full body workouts, also change exercises every day of the week.
Example for your back :
Monday 5 sets 10 reps shotgun rows.
Tuesday 3 sets 5 reps pull ups.
Thursday 5 sets of 5 reps dumbbell rows.
Saturday 5 sets of 20 reps rear delt flyes.