Today, I received some great news.

My English teacher told me that I have been selected to take the Cambridge exam at the end of the year, t. This exam is really important because it couldwill give me an official level of English, a Cambridge accredited level of English.

And that is well valued onby many colleges in Chile.

Now I need to improve many (abilites/skills) ,numerous competencies like for example, my pronunciation, since m. My teacher told my that it's a weak developed ability on me.one of my weak points.

My English teacher told me that I have been selected to take the Cambridge exam at the end of the year, t. This exam is really important because it could givplace me at an official (proficiency) level of English, a Cambridge -accredited level of English.

(1) I changed one of the commas into a period, because the way you used it is a little awkward. (You could also use a semicolon.) Commas by themselves aren't typically used to join two separate sentences; if you want to use the comma, you should pair it with a conjunction like "and": "...at the end of the year, and this exam is really important..." Even then, though, the sentence is a little too long, so you might want to break it up into two anyway.
(2) I tweaked your wording slightly to make it sound more fluent.

And that is well valued onby many colleges in Chile.

The correct word to use here would be "by" or "for".

Now I need to improve many (abilites/of my skills) , for example, my pronunciation, since my teacher told mye that it's a weak developed abilityness onf mine.

"many skills" isn't technically wrong, but I think in this context it sounds a little more natural to say "many of my skills".

Today I received great news.

My English teacher told me that I have been selected to take the Cambridge exam at the end of the year, this exam is really important because it could give me an official level of English, a Cambridge accredited level of English.

And that is well valued onfor many colleges in Chile.

Now I need to improve mana lot of my (abilities/skills) , for example, my pronunciation, since my teacher told mye that it's a weak developed ability onpartment for me.

But he also told me he thinks I can easily improve my abilities to reach B2 level.