(Fun fact: In older times, pink was for boys and blue was for girls.)
What's necessary to remember when writing from the point of view of a boy when you're a girl, or a girl when you're a boy, is that we are all human. Remember that, and the fact that most stereotypes don't apply to everyone, and you're golden.
Indeed, there are certain character traits that most girls seem to share, and some that most boys appear to have in common (guys aren't going to care about minimizing their pores and girls most likely won't find farts funny) but the cool thing about your character is that it's your character! So he or she can act however you want them to act.
You want to make your character realistic? The best way to go about that is to:
Ignore over-the-top stereotypes, like that guys only ever think about getting laid or that girls are only irrationally mad during their periods.
Read books told in the viewpoint you're going for to get a general idea of things (remember not to be a copycat though!) For girls, I'd recommend anything by John Green, and for guys...well guys, it shouldn't be too hard to find a YA book with a teenage girl narrating.
Write how you think your character, regardless of gender, would write.
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Writing Mistakes You're Making
NezařaditelnéA self-help book for beginning writers, covering everything from grammar to plot with a dash of humor to keep you interested and learning! (The best part? Anything you already know, you can just skip over.)