You well understand the basic essay format and have done a good job of organizing this one and presenting your ideas.
I have indicated some errors in the text and in my notes (referring to the underlined portions) below:
Where should we live? In a small town or in a big city? [The underlined is not a complete sentence-- connect it to the previous sentence.]
This question is really difficult to have a perfect answer. As far as I’m concerned, I would rather live in a big city than live in a small town. The three biggest reasons are the job opportunities, the services and the communication.
[You will write a better essay if you avoid all personal references (‘I think’, ‘I feel’, etc). Rephrase as: It is more convenient to live in a big city… Make the essay impersonal and it is automatically more formal.]
With no trouble, we can realise there are much more jobs in a big city than in a small town.
[Eliminate the underlined. Don’t pretend to give the readers’ abilities. Just get directly to the point: ‘there are more jobs…’.]
In a big city, the jobs include lots of areas, such as services and trades,… But in a small town, all the jobs are centred upon the farm and the field,… In addition to the job opportunities in a big city, we meet many people, so the chance for us to make friends and co-operate with them is really big. Thus, many people want to live in a big city in order to develop their careers.
[Lots of and really are too casual for essays. Do not use ellipses like that in an essay-- use and before the last item in a list or etc after it.
The services in a big city are always better than in small towns. First,they are more convenient. Most hotels, hospitals, zoos, cinemas and restaurants,… are concentrated in the big city; thus a farmer or a person who lives in a small town will have to go a long road to use the services. Furthermore, they give more thoughtful care to the customers that the services in a small town don’t have. For example, the entertaining areas in a big city are open until very late but in a small town, they close when it is only about 8 o’clock.
[Do not use contractions in essays: change don’t to do not.]
For people, meeting others and joining social events is very important and exciting. Certainly, in a big city, we can meet many more people than in a small town; have more neighbors. In a small town, each farm is very large so people have few neighbors. Moreover, the social activities in a big city are very diverse and ebullient. So, people will be excited. This is a really big reason that the population in a big city is always larger than the population in a small town.
[This last sentence adds nothing to the essay-- it is just filler. Cut it. Be sure that you are always building the essay with each sentence.]
In conclusion, I prefer to live in a big city because I like a busy life and the competition there. Some people will say that in a big city the enviroment is very polluted. But I do not mind it because I have hospitals and equipment to protect my health.
[As before, change the personal references to general ones: ‘it is better to live…’; ‘there are hospitals…’.]
[i]Good luck on your next effort.