How can I gain weight fast in a healthy way?!

If you want to put on weight, you should work out, to insure that you
put on muscle and not fat. Healthy weight gain, just like healthy
weight loss, takes time and
requires a conscious effort to apply good habits.
Recommended Ways to Gain Weight
Have meals with the right balance of proteins, carbohydrates, and the
right kinds of fat (such as unsaturated and monounsaturated fats,
olive oil, canola oil, pistachios, almonds and walnuts). Heinemann
suggests the following ratio: 60%-70% carbohydrates, 10%-15% protein,
and a small amount of fat.
Eat foods higher in calories, vitamins, and minerals, as opposed to
higher in fat or sugar.
Pack more nutritious calories in each serving. For example, you may
add grated cooked eggs to mashed potatoes, ground chicken to soups and
gravies, cheese in casseroles, eggs, and soups, and nonfat dried milk
in soups, shakes, milk, and mashed potatoes.
If you get too full too fast, try having more high-calorie foods or
slices of foods as opposed to consuming the whole thing (raisins
versus grapes, granola and Grape Nuts versus corn flakes, mango slices
versus the whole mango).
Limit drinking beverages to a half-hour before and after a meal.
Drink mixed juices (apple/berry, peach/orange/banana as opposed to
one juice beverages) for a higher calorie intake.
With moderation, you may add in good fat sources to meals such as
nuts, avocado, olives, and fatty fish (salmon and mackerel).
Snack in between meals. Nuts, dried fruits, and yogurt are good
options, but it's also important to find nutritious foods that you
will enjoy.
Have a nutritious snack before bedtime, such as a peanut butter
sandwich.

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eat alooot of peanut butter, nuts, cheese, avacados, sausage, red meats, etc.
skip the veggies, fruits, chicken, etc for now.
and add butter, gravy, syrup, cheese,sugar, cream etc to anything you can.
also, working out to build muscle would be good.

Drink nutrition drinks. You can get them at the grocery in the medicine section. Hope this helps!

Work at a hospital and give them drinks too ppl that need to gain weight

I wouldn't get so stressed about it because stress can affect you too.
Just plan your meals more carefully. Lots of carbs for breakfast this is the fisrt meal of the day and your body will benefit from a decent meal, for the secnond meals just add and extra 300 calories to your meal (maybe have a desert or just bigger portions of what you normall have)
Don't try to do it quickly just build it up slowly.
You can also get build up drinks from the health food store, they are like a milkshake but contain loads of vitamins and more importantly in your case calorific substances to help you maintain or raise your weight these shakes were originally produced for children in poorer countries that couldn't get enough food, but obviously there are a place for them in the health food stores for people in your situation too.
Get better soon : )