I always pack an apple for lunch, but when I do eat it I realize it is always soft, brown, and not in a good condition, also, the apple is not cut, how do I prevent from my fruits getting oxidized?
If you want to make sure you get a fresh apple every time, you'll have to make sure you're taking care of the fruit at every step.
1.) Make sure you're buying good fruit.
While organic fruits are widely desired, they also tend to rot quicker due to the lack of preservative coatings. Cheaper apples are coated with layers of food-grade wax, meant to protect the apple and keep it fresh for longer. Of course you should also be checking the skin of the apple for any discoloration, spotting, bruises, or holes. Also give each apple a turn and squeeze with your hand, so you can check for any soft spots that might indicate bruising beneath the surface. Taking a moment in the store to consider the quality of what you're buying will save you tons of hassle in the long run.
2.) Make sure you're storing it right at home.
Apples keep best in the cold. They will last longest in temperatures between 30 and 35 degrees Fahrenheit. They will also benefit from high humidity levels, meaning that they are a perfect candidate for your fridge's crisper drawer, if you happen to have one. If not, simply store the apples in a dark area of your fridge, preferably in a sealed container. This Insider article even suggests storing the apples in plastic bags with damp paper towels, in order to increase the humidity. You should also take care to store the apples away from your other produce, as many vegetables produce ethylene, a chemical that promotes ripening in nearby fruits.
3.) Make sure you're storing it right when you bring it for lunch
When you take your apple for lunch, leave it whole for as long as possible. if you intend to slice it, do it right before you eat it. This will prevent any unwanted browning. Also make sure you're keeping the apple as cool as possible, whether this means putting it in a fridge, or simply keeping it away from your lunch and thermos.
Probably it is just bruised, not rotten. When you pack it for lunch, put it in a box or wrap it in something (cloth, bubble wrap) to prevent this, and keep it away from any warm things you've also packed.
Some fruits (at least including apples and avocados) start to brown because they are exposed to an environment (the air) that is more alkaline than the fruit itself. Brushing the surface of the fruit (or guacamole, etc.) with a small amount of acidic solution (lemon juice, lime juice, vinegar, ginger ale, Vitamin C (ascorbic acid), etc.) diluted in water may prevent this browning from occurring. A bit of experimentation may be required to determine a concentration that is sufficient to stop the browning but not enough to impart an unpleasant taste.
This page has some discussion of various alternatives. There are many other pages with similar information on the internet. (Side note: The page also suggests that the sharpness of the cutting tool you use may have an effect.)