To build up stamina for running, you want to slowly increase your running load. This could be how many days per week you're running or how much time or mileage you're running with each run. Maybe you're running two to three days a week right now. Say you're running about three miles each day. Maybe you can run four days a week, three miles each time, or maybe those two to three days a week you can add a half a mile. But challenge yourself. Increase your run, increase your distance.
Over the course of about four-six weeks, maybe even eight weeks, you'll start to notice a difference and you'll start to notice that that six or eight mile run is not as challenging as it was a few weeks ago. Just keep challenging yourself. Increasing the time that you're running. Increase your mileage. As you increase your time or you increase your distance, it'll feel uncomfortable at first. But if you stick with it, if you stay consistent with your plan you'll start to notice that, it becomes easier over time as your body adjusts.