Hi, there.

I am quite new here and it's my first time to posting something in English.

I used to post something in English to improve my English writing skills on Lang-8 but they willhave closed.

That's the same reason I am here! I used to use Lang-8 as well, but since the company is no longer supporting the website, I've had to look for an alternative.

Since I noticed that, I amhave been looking for a new place where I can keep writing in English.

I hope I can meet lots of people here and help oueach other studying languages each other for years.

Hi there.

Hi!

I am quite new here and it'this is my first time to posting something in English.

If you include a Japanese version of your text, it can help me understand how you would describe this in Japanese and where the grammar-semantics confusion might be. (A little bit selfishly, it also helps me learn more about how people describe things in Japanese.)

In any event, "to post" does not mean actually doing the thing, that's why the form "posting" is necessary. An alternate wording is "this is my first time trying to post something in English" and in that cause the -ing you're doing is "trying" and "to post something" is a description of what you're trying.

I am still exploringfiguring out how to use this website.

You can also say "Exploring the website," but while "exploring how to use" isn't technically wrong, it's not a typical expression.

I used to post somethings in English to improve my (English) writing skills on Lang-8, but they will close soon.

something -> things: "I used to" suggests a habitual, repeated action, but "something in English" refers to only a single thing which could only have happened once. Also, clearly you don't mean "only once" here.

"but they will close.": Again, technically there is nothing wrong with this, but it feels pretty awkward to say "they will close" without clarifying "soon," "at some point," "in the future," etc..

Since I noticlearned that, I amhave been looking for a new place where I can keep writpracticing in English.

noticed -> learned: I double checked the definition of "notice" and Webster says "to become aware of (something or someone)." The subtlety to note is that you "noticed" the announcement, however the meaning you take from that announcement is what you "learned."

"I noticed Jim was wearing a new shirt with text on it. Reading his shirt, it says 'if you can read this, you're too close.'"

writing -> practicing: This is just for precision. You can write English anywhere, but here you can get feedback which provides better learned. In other words, this site facilitates practice.

I am quite new here and it's my first time to posting something in English.

I am still exploring (how to use) this website.

It would be more natural without “how to use.”

I used to post somethings in English to improve my English writing skills on Lang-8 but they will close.

Since I noticed that, I am looking for a new place where I can keep writing in English.

In this sentence, “since I noticed” means “because I noticed.” If you want “since I noticed” to mean “from the time when I noticed and continuing even now,” you will need to change “I am looking” to “I have been looking”:
“Since I noticed that, I have been looking….”

I hope I can meet lots of people here and help outhat we can help each other studying languages each other for years.