I joined now(only) just (now) joined this site.

The words in parenthesis are optional; I think they mostly just add more emphasis
Adding in "this site" at the end helps it feel less blunt

I mainly use English mainly in emails at my work.

Sometimes I make our customers confused due to my poor English.

I hope I canto exchange emails smoothly with customers.

You could also say "I want to be able to exchange emails smoothly with customers."
Saying "I hope I can" implies something inevitable that will occur that you hope to overcome or have go well. It can imply a sense of being unprepared for what's about to happen.
"I hope to" and "I want to" are what you want for something you aspire towards; "want to" is more common but both are accurate.

Is it ok to post here an email that I want to send to a customer?

It's somewhat implied that you would be posting it to this site, but if you wanted to add the "here" to avoid ambiguity I think it sounds best to add it after "email".
Also, when it comes to singular or plural in this sentence, it depends on intent - if you had one particular email in mind that you wanted to post, then singular is fine, but if you plan on regularly posting emails then changing it to plural ("an email"->"emails"; "a customer"->"customers") would be better.

I want to have my message(s) checked ifto make sure it doesn't make athe customer(s) confused. ¶

Whether or not you make the nouns in this sentence singular or plural depends on if the previous sentence is singular or plural, so it could go either way.
However, if you phrase it differently you could have a bit of both: "I want to have my messages checked to make sure a customer doesn't get confused."
In this particular phrasing, "a customer" is used to refer to an unspecified somebody who happens to be a customer, which I think is why it's singular.

I just joined now.

"now" usually means "right now" while "just" means it happened a little before now.

I mainly use English mainly in emails at my workin my work emails.

In English, adverbs such as "mainly" are best placed directly before the verb it modifies, in this case "use".

Try to use the form: [adverb] + [verb]

Sometimes I make ourpeople confused due to my poor English.

"our" is a possessive pronoun. This sentence sounds best when you directly state who you confuse.

I hope I canto exchange emails smoothly with customers.

"I can" is grammatically correct but "to" or "to be able to" sounds more natural because it refers more to a possibility in the future.

Is it ok to post here an email that I want to send to a customer?

Putting "here" at the end of the sentence sounds more natural to a native English speaker, but where you put "here" is perfectly fine.

Adding "that" makes this sentence easier to read, but you do not need it.
See this page for more information on reduced relative clauses (e.g. "I want to send to a customer"): https://www.englishgrammar.org/reduced-relative-clauses/

I want to have my message checked ifso that it doesn't make athe customer confused. ¶

"the" works better than "a" here because you are referring to a specific email to a specific customer.

Thank you for your cooperationhelp.

"Thank you for your help" is the standard when asking for help.

If you were working on a team you might instead say: "Thank you for your cooperation".

I just joined now.

I mainly use English mainly in emails at my work.

This placement for the adverb "mainly" is a bit more natural.

Sometimes I make ourthers confused due to my poor English.

I hope (I canwill be able) to exchange emails smoothly with customers.

The "I will be able" part isn't necessary; it sounds fine both with and without.

Is it ok to post here an emails I want to send to a customers here?

Since you aren't talking about one specific email or customer, you would use the plural to indicate that you are talking about emails and customers in general.

I want to have my messages checked if itto make sure they doesn't make a customer confused. ¶
confuse the customers.

Thank you for your cooperationhelp.

"help" sounds more natural and less business-like than "cooperation" here.