Fitness will make you gain weight to a great extent. Losing weight and fitness is another story.
A very thin person, when those joints in your arms, chest and legs are no longer scary and look thin, it means that you have successfully gained muscle. Not to cover with fat, but to make your muscles swell up and become stronger.
The first thing is to eat more rice, so that you can have the strength to exercise. You can eat more beef, which is good and will make your meat stronger. Because you are thin, you should mainly eat meat, beef, chicken breast and fish, and eat more of these things.
You must eat eggs and drink more milk for breakfast. You can eat an egg, but you can drink two or three glasses of milk. Milk will nourish your body and make you stronger. For vegetables, cabbage and potatoes, eat less, although other fitness will eat these things, but that is generally weight loss fitness. You can eat more broccoli instead of salt and oil.
If you train, don't train too much at first, otherwise it won't last. Because you are too thin, some sports are not suitable for you. Do normal running and push-ups first. If you are free, swim more. Wait until your body is fuller, and then do some strenuous exercise.