This is a pretty good report! I have not amended any stylistic aspects, as I think it has its own style - I have just amended those grammatical aspects which jumped out at me.

Report about smartphones

This is a report about mobile phones I have to do for my English class. I'm not sure if it's a correct report or maybe it seems more like an article :(

This report describes the choices for a person who wants to buy a new smartphone.

Choosing the correct size

The choice of the screen size must be done depending on the intended use. Normaly mobiles with big screen size are better for users who want a smartphone for surfing in the internet. However a big size is not always an advantatge because it reduces battery life increments the battery waste, and besides the mobile phone becomes more uncomfortable due to the increased increment of the size and weight. 

iPhone Android, or Windows Phone - which is the best choice? 

One of the advantages of the iPhone is the fact that it has a specific hardware designs for its Operation System, and as a consequence, the performance increase improves significantly. On the other hand, Android has become very popular in the lasts few years due to the variety of prices and the number of free aplications. In the last place, the Windows Phone has been the last to arrive on the scene but it has the support of Nokia which is a mark of quality.

Price - how much can I afford?

Clearly, one of the determining factors determinant when choosing a smartphone is the price. The price will determine the features of the mobile phone, the quality of the camera, the display or the processor. However, the price affects the giving use or the choice of the brand, iPhone, Android or Windows Phone definitely affect the device price. - This sentence is not entirely clear and could be expressed more simply.

In conclusion, the iPhone is for people who want a good device without worrying about the price. Android give a large quantity of choices due to the different prices and features. Finally, the Windows Phone benefits from its link with Nokia, a symbol of quality, despite the lower number of applications offered.