TOEFL Writing #10: Do you prefer to eat out or eat at home?

I think that the best option is eating at home for several reasons. There are many reasons why eating at home is the best choice.

Firstly, when you prepare foods, you know what you are eating. This means that your meals will be healthier. In fact, many times in fast foods they use are often prepared using too much oil or sauces, and a lot of food it is even fried. If you eat in restaurants every day, you risk to gaining weight easily. In fact, there are a lot of "hidden calories" in such meals: even in a salad plate, many sauces could be added and calories easily increase even a salad can have many sauces added to it that significantly increase the calorie content. Moreover, in fast foods you can rarely eat vegetables or fruit, which are instead very important for your good health.

Secondly, preparing food at home means that you can save a lot of money every month. In particular, This is particularly important if you're still a student. Even if you have a well paid job, why do want to you spend money to have low quality meals by eating in stand foods?

Not sure exactly what you mean by "in stand foods", maybe "takeaways" or "ready meals"? Probably better to phrase it thus: "Even if you have a well-paid job, why waste your money on takeaways?". 

Thirdly, preparing food means eating healthy healthily, which is not just something related to your weight, but it also have has consequences on for your brain. You'll hardly be concentrated concentrating during lectures or at work if you've eaten too much. Furthermore, your whole body will suffer if you only eat junk food only. Remind Remember that your health depends on 70% by on what you eat, and just 30% by on physical activity. Hence, even your performances are in everything is influenced by this. 

Your first point was all about cooking for yourself being more healthy so this paragraph makes it seem as though you haven't got your argument logically organised.

When I was a bachelor student an undergraduate, I used to eat at in the university dining hall. This was because I thought that cooking was just a waste of time, and even thought that there was no difference from preparing food by myself and eating at food stands between the food there and meals I cooked myself.

I think I get what you mean by "food stands". It is not wrong but it is an uncommon expression. At least not one I've ever come across. The word stand is generally used in conjunction with a specific type of food. For example hot-dog stand, beer stand or ice cream stand. You are trying to explain a contrast so use "difference in x between y and z" or simply "difference between y and z". For example, "… that there was no difference in quality between food from a cafeteria and meals I cooked myself."

Now I always prepare food by myself, and I have noticed do notice the difference (mainly in quality).

By myself = alone. Myself = instead of someone else.

In general, it is not true that cooking requires more time. In fact, many times I often had to queue in the dining hall; hence, the amount of time is almost the same for the same amount of time it would have taken to cook for myself.