Don’t capitalize the first word of a dialogue tag that follows the dialogue. There is no reason to capitalize a pronoun unless it’s the first word of the sentence or it refers to a deity. (Occam’s Razor, kids… Don’t try a “But how do you know this character isn’t a deity, huh?” argument. Believe me, incorrect capitalization is far more common in fiction manuscripts than are deities of any gender.)
Unless the dialogue ends with a question mark or exclamation point, use a comma (inside the quotation marks) to separate dialogue from its tag.
You know I’m not adverb-phobic, right? (Remember how I mock people who insist that all adverbs are Ultimate Evil of Evil-Bad-Writing-Ness and must never be used?) On the other hand, I am opposed to using unnecessary, inappropriate, or redundant words, and since whispers are soft (unless specifically stated otherwise — it is possible to whisper harshly, for example), softly doesn’t need to be there.
“You’re one of them now,” she whispered.