Explanations

TAKE A PHOTO= It's a way of saying: why are you staring at me? (stare= look intensely at someone. Staring a stranger is rude!)
IM KIDDING!= I'm joking, I'm not serious.
STACEY= her name.
JASON= his name.
NICE 2 MEET U= Nice to meet you.
NICE TO MEET U 2= Nice to meet you too.

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Playing the game "noughts & crosses"= you have a grill with 3 columns and 3 rows. In turn, both players draw a cross (player 1) or a naught (= a zero) (player 2). The purpose of the game is to get three crosses or noughts in a row. The row may be horizontal, vertical or diagonal.
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I HAVE A SECRET
I WAS WATCHING U FIRST= I was watching you first (I had started watching you through the window before you noticed me for the first time)
DO U WANT TO MEET?= Do you want to meet? (= can we meet?)

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I GOT PROMOTED= I have been given a better post in the company.
WE SHOULD CELEBRATE= The verb CELEBRATE may be transitive or intransitive, so we can say "we should celebrate your promotion" or simply "we should celebrate". To celebrate is to have a party or some drinks or any other kind of fun because something good happened.
ABSOLUTELY!= Of course! Sure! (but more emphatic)
DO U WANT TO MEET= Do you want to meet?
THOUGHT YOU'D NEVER ASK= I thought you would never ask. This means that she really wanted him to say that but she wasn't very sure he would do it, or simply she was too impatient about it.
HI= hello.