I am also 25 years old, almost as tall as you, but heavier than you. I was thin a year ago. I have superficial gastritis and bloating. So I often can't eat much. In order to improve health: First of all, eat less meals to give the stomach time to digest and absorb, eat more high-protein foods, such as eggs, beef, milk and fruit as snacks, and eat something every two hours. Bodybuilders are all "3 points by practicing, 7 points by eating". This shows the importance of nutrition.
You have a bad stomach, so you should also pay attention to eating less and more meals, and remember not to overeat.
With good nutrition as the backing, you will have the strength to keep fit. You are thin, you can start with aerobic exercise, run more and jump rope for 2 months to improve your physical strength and adaptability.
When you have physical strength, you can do strength training. This is a process.
I suggest you go to the gym and see more people who practice well. You can ask them. They are also slowly practiced and will generally teach you.
I hope you have a strong body as soon as possible-