You should consume energy rich foods like nuts ,whole cereals, dals,potato, sabudana,banana, sweet potato, milk ,butter, cheese,yogurt,fruits, etc.
Drink 3 to4 lits water daily . Regular exercise.
If the weight loss post pregnancy has been drastic then please do visit a Dr and get certain tests done.If not then the reason behind weight loss could be breast feeding and the sudden change in your schedule which keeps you busy and you are unable to take care of yourself and the diet.Do not worry just follow the tips that are below.
Tips which can help you in gaining weight -
1)Eat small and frequent meals , if you are eating 3 meals in a day break them down to 6 and try and have meals every 3-4 hours.
2)Even though you might feel that eating a burger,pizza with some coke and mountain dew would help you in gaining weight ,Beware!!.These foods can make you fat in the long run.Eat good and healthy food by that I mean good amounts of protein,healthy carbs and good fats.You need to build muscle and not a storehouse of fats.
3)Eating protein rich foods i.e foods like milk,paneer,chicken,eggs,fish and cheese becomes important because without enough protein in the diet the entire food would end up turning into carbs and ultimately fats.
4)Eat calorie dense foods like nuts(almond,walnut,cashew,pecan nuts,peanuts) and good amounts of fruits and veggies.
5)If you have to eat carbs then make sure you choose complex carbs like oatmeal,brown rice,quinoa and barley.Do not have too much of simple carbs(candies,doughnuts,pastries,cakes),remember these shall make you plum and not healthy.
6)Exercise!!Yes weight training can be your best friend if you have to put on weight and build some muscles.Resort to compound exercises like squats,bench presses,chin ups,pull ups and dips.
7)Sleep well,by that I mean sleep for at least 8 hours and while you are sleeping the body is building up some muscles.
8)Do not skip meals as if you have to gain weight you need to eat frequently.
Last but not the least remember with muscles you would gain some fat ,but thats completely ok.Eat less of refined ,processed and high fattening foods and you would be left with a killer Physique.
Let me know if you need any further assistance and feel free to get in touch with me :)