Hi Niilo

Below are my suggestions.  I hope they are useful.  I wish you good luck for the future!

I have used English online for several years and this has led to the point where I can read everything I need to and completely understand it.

Alternative = I have been using...

Either way, I think your opening sentence implies that you have used English online for reading only, rather than for general communication. 

However, recently I've been more and more involved in language learning and now I'm looking for language partners to study Japanese with.

Alternative = ...I've become more...

My native language is Finnish, and because there's not as much Japanese people who want to learn Finnish than there is those who are studying English, I was thinking maybe I could try teaching them English instead of my native language.

..as many people

Alternative > ...but as there is much more demand for English from Japanese people than for Finnish...

First I have to be sure I know English well enough myself, of course, and that's why I'm now writing this notebook entry.

Of course, first I have to...

Alternatively > ..firstly...

English training

I have used  been using English to communicate with people on the internet online for several years and this has led to the point where I can read everything in English I need to and completely understand it. However, recently I've been more and more involved in language learning and now I'm looking for language partners to study Japanese with. My native language is Finnish, and because there's not as much are not many native Japanese people who want to learn Finnish than there is those who are studying English, I was thinking maybe I could try teaching them English instead of my native language. First I have to be sure I know English well enough myself, of course, and that's why I'm now writing this notebook entry. 

Be strict or even cruel with your corrections, please!

(I'm an English learner myself. Above is what I would write. Let's see how others write it.)