What exercise should I do to get slim in a month?
In general I like to recommend strength training but since you only have one month and you want to get slim in one month outside with no equipment, then your best bet is outdoor HIIT sprint or sprint interval runs. Sprints are one of the most effective fat burning workouts and they also really help you build muscular body. Look at a body of a sprinter then other athletes. They are the leanest.
Here is a great outdoor 20 min HIIT sprint workout you can do outdoor and preferably if you can find a hill but if you can't, then a flat surface should work as well.