I decided to join thea cram school yesterday.

Using "a" is better here. (I'm not sure why; it just feels right. :) )

The reason why I did this is because I found that after nearly two months of self-study; my efficiency of, I was not adequately absorbing all the knowledges that is required is inadequate necessary.

That means I'm not able toContinuing on this track, I would not be prepared before the recruitment for Starlex by the end of thise year.

The meaning is a bit vague. I believe the intention is that if you continued with self-study, then you would not be prepared for recruitment.

This problem has bothered me for almost two months, so I'm glad that I don't have to worry about this problem from now onanymore.

Hope I'll be able to concentrate after starting the courses ofat cram school.

I decided to join the cram school yesterday.

You could also say "cram school" in general if it's not important to specify a particular cram school.

The reason why I did this is because I found that even after nearly two months of self-study; my I was inefficiency oft at absorbing all the knowledges that is required is inadequatthe required knowledge.

The original version was on wordier side. When talking about knowledge collectively, you can just use "knowledge"

That means I'm not able to be preparedady before theStarlex recruitment for Starlex bys new hires at the end of this year.

This problem has bothered me for almost two months, so I'm glad that I don't have to worry about this problem from now on.

HI hope I'll be able to concentrate after starting the courses of cram school courses.

Starting off with "hope" would work if you were just talking in a casual setting since the "I" would be implied, but in a written sentence you should add "I". Also, "the courses of cram school" can be condensed to "the cram school courses".