Hmmm...that's a good questions actually!
I have my own way of writing for my manga, but everyone has their own style.
You can always write it out in script form like follows:
Tim: Hey...seriously...who ate my mango
Jon- (looks uneasy) Um..mango...?
Tim: (looks suspiciously at Jon) Dude...you know which mango.
-Jon makes his way out of the room. Tim looks pissed.
LOL Pretty basic, but you'll get your ideas down quickly and there's no extra reading to get through
I hoe this helps!