Earned 2 yen

　About a year ago, I registered myself as a Android developer to sign up for the Play store. And few months ago, I registered my first app with the Play store. This app was a countermeasure tool for qualification tests that is related to information technology. The target of this app was very niche, however, I created this for myself. Also, since there are almost no advertisement on this app, it's really difficult to earn money. I today visited to the AdMob page which shows advertising revenue after a long interval. Then, I found that I had earned 2 yen by advertisements of my app. That's one small step for many people, one giant leap for me.

• And few months ago, I registered my first app with the Play store.
• And few months ago, I uploaded my first app on Google Play store.

• The target of this app was very niche, however, I created this for myself.
• The target group of this app was very niche, however, I created this for myself.

• Also, since there are almost no advertisement on this app, it's really difficult to earn money.
• Also, since there are almost no advertisement for this app, it's really difficult to earn money.

• Then, I found that I had earned 2 yen by advertisements of my app.
• Then, I found that I had earned 2 yen by displaying advertisements (or: ads) on my app('s) screen.

Interesting!

• About a year ago, I registered myself as a Android developer to sign up for the Play store.
• About a year ago, I registered myself as an Android developer to sign up for the Play store.

• And few months ago, I registered my first app with the Play store.
• And A few months ago, I registered my first app with the Play store.

  It sounds better here not to use "and" at the beginning of the sentence. (Maybe like you said in your previous post, it's really true that Japanese tends to use a lot of 「そして」など that don't get used as often in English!

• This app was a countermeasure tool for qualification tests that is related to information technology.
• The app was a countermeasure tool for qualification tests that is related to information technology.

  Either "this" or "the" is good

• The target of this app was very niche, however, I created this for myself.
• The target demographic for this app was very niche, however -- I created this for myself.

  面白い！
  
  The punctuation I used here ( -- ) is called the "em dash" -- basically it's a long pause! I wonder if something similar exists in Japanese? (I am interested in punctuation differences between Japanese and English in general! I'm sure there are a lot of differences and I don't think lots of people spend time learning about them.)

• Also, since there are almost no advertisement on this app, it's really difficult to earn money.
• Also, since there are almost no advertisements on this app, it's really difficult to earn money.

  (I think the previous commenter is confused, and thinks you are advertising for this app, whereas in reality you are placing ads in your app to generate revenue)

• I today visited to the AdMob page which shows advertising revenue after a long interval.
• Today I visited to the AdMob page, which shows advertising revenue after a long interval.

  Good! Correct use of "which" vs "that!"
  
  I'm thinking now, and I think I can give you one more rule: always use a comma before "which."
  
  I like using black pens, which are darker than blue pens.
  I like using MacBooks, which are better than other laptops.
  I live in Japan, which is an island.

• Then, I found that I had earned 2 yen by advertisements of my app.
• I found that I had earned 2 yen from advertisements on my app.

  Previous commenter's suggestion is also good. Again, though, I think the use of "then" here is incorrect and you are better off without it!

• That's one small step for many people, one giant leap for me.
• This sentence is perfect! No correction needed!

Nice!