Today, I onlywill talk about Mormon missionaries.

I once asked a Baptist Pastor, how does what he thinks of Mormonism.

What he thinks about Mormonism = also ok

He said that this group is only coloosely related withto Christianity, although they claim they believe in God.

Sorry I didn't understand what you meant, I corrected this as if you were trying to say they aren't that close

Today, I'll only talk about Mormon missionaries.

In Canada, where I a'm staying around, most of the missionaries are from the USA, and some of them proudly claim their familyies have been in the faith of Mormon faith for over a hundred years.

families because plural, families of missionaries

if "their family" it implies they all are from the same family

Boys and girls from these families would be dispatched worldwide to serve as missionaries for about two years, and mainly in Canada, Australia and New Zealand.

dispatched needs "be" as something other than them caused them to be dispatched

BThe boys are usually in clean formal suits, and shining leather shoes, and are always courteous, and look. They energetic and active, while the girls are in dresses and look elegant.

commas to group the clothes together. It's better to not use so many "and"'s consecutively though

They often push and follow every potential new Mormon new member.

the order of adjectives is very difficult for even native speakers, but it should be this way (you'll get it with time) generally the more specific the adjective, the closer to the noun

Moreover, these missionaries pay half of their expenses themselves whilen at work, andwhile the church pays the other half.

"while" to show it's being done at the same time

On their official website, they do not solicitly donations publicly, but encourage making private support.

Support is also a verb, so you can just say encourage support. No such thing as "making support" in English

That's what I know about Mormon missionaries.

I once asked a Baptist Ppastor, how doeswhat he thinks of Mormonism.

He said that this group is onmerely coated with Christianity, although they claim they believe in God.

merely: Like only but with negative connotation

Today, I will (only talk OR be talking) about Mormon missionaries.

In Canada, where I am staying around, most of the missionaries are from the USA, and some of them proudly claim their family have been in the faith of Mormonism for over a hundred years.

Boys and girls from these families would be dispatched worldwide to serve as missionaries for about two years, and mainly in Canada, Australia and New Zealand.

I'd prefer "Children" over "Boys and girls" in this sentence.

BThe boys are usually in clean formal suits, and shining leather shoes, and are always courteous, and. They look energetic and active, while girls are in dresses and look elegant.

Use "and" less and make sentences clearer by splitting them.

They often push on to and follow everany potential (new Mormon new member.members. OR Mormon converts.)

On their official website, they do not solicitly donations publicly, but encourage making private supportcontributions.

I once asked a Baptist Pastor, how does what he thinks of Mormonism.

Alternative:
... what he thinks of Mormons.

He said that this group is only coated with Christianity, although they claim they believe in God.

Alternative:
... cloaked in Christianity, although they claim they believe in God.

Today, I onlywill talk about Mormon missionaries.

Boys and girls from these families wouldare dispatched worldwide to serve as missionaries for about two years, and mainly in Canada, Australia and New Zealand.

Boys are usually in clean formal suits, and shining leather shoes and. They are always courteous, and look energetic and active, while girls are in dresses and look elegant.

They oftend to be pushy and follow every potential Mormon new member.

On their official website, they do not solicitly donations publicly, but encourage making private support.