How Can I Increase My Stamina ?

To premise all of this, I'm not a doctor or specialist at this. This is just what I did.

Start by developing a baseline of endurance. This is an easy way to build that endurance. Choose a certain distance, I recommend at least a mile, and walk it quickly, but at a constant pace that you can maintain. Time yourself as you do.

Then walk it again the next day, and try to take off one second for every minute of your previous time. So if you chose 2 miles and walked it in 25:36, try walking it 25 seconds faster. Your goal would be 25:11. Once you meet that goal, repeat the process. It's not too difficult, because you only have to go a little faster each time. 1 second off of every minute. You'll probably be able to meet each goal within a few tries.

After a while, you'll find that the only way to meet your new goals is to start a slow jog. Once you can slowly jog the entire distance you chose, then you can start building more advanced stamina. Inject a short sprint into your jog. Maybe just 10 paces or so. Do this without ever breaking your stride, and don't ever go slower than your slow jog pace if at all possible. Once you feel like you've recovered from the sprint, do another sprint.

Each day you succeed, at one more sprint than last time into your routine until you're satisfied with the number of sprints per mile. Once you reach the number of sprints you want, start increasing their length. Maybe up from 10 paces to 12 paces. Do this each day that you successfully complete your routine. If your sprints get very long, you may have to reduce the number per mile. The balance of sprint length to sprint number is up to you.

I used this method and became able to run 2 quarter mile sprints per mile, over a distance of 2 miles, while still jogging the other quarter miles. This builds a combination of endurance and stamina at the same time. 2 miles may not seem that long, and it isn't really, but I didn't personally have time to make much longer runs. As I got faster though, I eventually kicked it up to 3 miles. I wouldn't personally recommend running more than about 20 miles a week though. I think that overrunning could be bad for the joints. Walking however, can probably be done as much as a person wants with very little risk to the joints.

Remember to eat healthy, and get good rest. Not doing those will make progressing much more difficult. And if you feel deep or sharp pains in your chest or joints, dizziness, numbness in your extremities, or suffocation during your exercise, then ease up. Your body can break if you push to hard. Over time you'll learn which aches and pains are normal and can be endured without great harm.

Treadmill/cycling.

Avoid foods impacting your lungs/smoking.

Eat healthy food that boosts stamina.

no need to! On internet you can pretend that you're super fit!