Prepare for going out.

Prepare going out.
Prepare to go out.

(Which one is correct? Are all correct?)

Give me three garlics.
Give me three pieces of garlics.
(Which one is correct?)

Did you Unplug the electric cord from the socket after cooking? Don't do that. You should keep the electric cord pluged in the socket after cooking rice. Then the rice will stay warm in the rice cooker.

(Is this passage grammatcial?)

Let's attend the English service.
Let's join the English service.
Let's take part in the English service.

(Are the three the same? Do they have any diference? Which expression is commonly used?)

Prepare for going out.

Prepare going out.
Prepare to go out. <~~correct!
(Which one is correct? Are all correct?)

Give me three garlics.
Give me three pieces of garlics.
(Which one is correct?)
Neither is quite correct. It should be -- Give me three pieces of garlic.

Did you unplug the electric cord from the socket after cooking? Don't do that. You should keep the electric cord plugged in the socket after cooking rice. Then the rice will stay warm in the rice cooker.
(Is this passage grammatcial?)
All is fine except for a couple of letters that I put in bold above.

Let's attend the English service.
Let's join the English service.
Let's take part in the English service.
(Are the three the same? Do they have any diference? Which expression is commonly used?)
All are pretty much the same. The word "attend" implies going and sitting and listening. The word "join" could imply going into the service after it has already begun. The phrase "take part" implies being more active than just sitting and listening; perhaps singing, too, and/or taking communion.

All three sentences in the first set are correct and have similar meanings. They all convey the idea of preparing oneself for going out.

In the second set of sentences, "Give me three garlics" is grammatically incorrect. The correct sentence would be "Give me three pieces of garlic," as garlic is a non-countable noun.

The provided passage is grammatically correct; however, there is a minor typo. "Plugged" should replace "pluged." The passage suggests that you should keep the electric cord plugged into the socket after cooking rice to keep the rice warm in the rice cooker.

Regarding the third set of sentences, all three are correct and can be used interchangeably. They all mean the same thing, which is to participate in or be present at the English service. However, "attend" and "take part in" are slightly more formal expressions compared to "join," which is commonly used in casual contexts.