Many native speakers have trouble with "not only...but also." The key is to make sure that the parts of speech that follow each set of words are formatted the same way. In English we call this parallelism. "I was selected to be not only a class leader but also a cheerleader." Notice that each part of the "not only...but also" construction is followed by a noun phrase. You could write the sentence in other ways: "I was selected not only to be a class leader but also to cheerlead." "I was selected not only as the class leader but also as a cheerleader." "I was selected not only to be a class leader but also to be a cheerleader." (This one is not as strong because of the repetition.) This next example would be common to see, but incorrect: "I was not only selected to be a class leader but also a cheerleader." Notice how in this sentence the verb "selected" follows the first set of words but "a + noun" follows the second. They are not parallel.

I would say it in a few of ways: Not only was I selected to be a class leader but also a cheerleader I was not only selected to be class leader but also a cheerleader I was selected not only to be class leader but also a cheerleader

"Not only was I selected to be a class leader, but I was also a cheer leader" <--- Formal written way "I was selected to be a class leader, but not only that I was a cheer leader also " <--- Less formal, more commonly spoken than written