[CAUTION: I am not a teacher:take the advice and or corrections offered in this post at your own risk.
If you doubt the information, please get a qualified opinion from one of the teachers on these forums.]
Please note the disclaimer. There will be two kinds of corrections:
Technical[in red, remove your red words, replace them with words in the brackets (pay attention to, but do not write words in parentheses)]
Style[consider changing, adding, or using this information]
Please check that the meaning of your article has not changed after any changes you make, if you have questions please ask and I will try to answer promptly (of course anyone else is welcome to as well).
The internet, one of the latest discoveries[technological advances] of the 20th century, has become part of everyone's vocabulary nowadays. Whether we like it or not, it is turning more and more into a social phenomenon, especially among teenagers.
[consider line breaks between your paragraphs]
However, it cannot be denied[I would avoid the passive voice and say: "one cannot deny"] that it provides a lot of facilities[it facilitates (makes easier) many things]. I was a little skeptical towards it at first, but now I am aware of the positive effects it has proved to have on my life. The internet can be a source of useful information, which you[I would say "one", I was taught not to use "you" in an essay] can find and download by accessing an appropriate[appropriate seems to carry moral judgement, I would say "related"] website. For example you can study on line[online]and in this manner you can save a lot of energy and you don't need to borrow books from public libraries or you don't need to buy a lot of books. So[For these reasons] the internet is absolutely convenient.
It is partly[why partly?] true that the internet world contains a huge number[amount] of information[,] so sometimes when you are looking for a common knowledge you can[may] lose[waste] too much time to find[searching for] it. However, with this charming[in this [wonderful]] world you can keep in touch with everyone although[even if] he /or she lives across[in another part of] the world. I remember that last year, when I lived in Spain, I kept in touch with my boyfriend every single day thanks to this amazing invention["technology" perhaps]: the internet.
Unfortunately, somebody[some people] use the internet exces[s]ively, mostly children and teenagers, which can lead to [them] finding it harder and harder for them to adapt to[function in/adjust to][the] real world. [Overall,]This world[The world of the internet(?)] is a great one if used properlyand with this[its] aid you can save too much[quite a lot of] time.