As a native English speaker, I don't necessarily use "dude". It's an Americanism. Maybe if I wanted to use some mock-American for fun...?

You can use "dude" to start sentences, regardless of whether you're talking to a girl or a guy. "Dude, what is up with the heat lately? Sheesh it's hot out" "Dude, wanna get some food?" "Dude I'm diggin' the shoes" You can use "dude" to address a specific guy "Hey dude, what's up?" "Look at this dude, he thinks he's pretty cool" "Dude, please stop" It can also be used as a reaaaally informal adjective, to mean "dude-like" or "dude-esque" "Yeah that's pretty dudely of you to say" You can replace "dude" with "bro" or "bruh" and it works the same in all cases. All are pretty equally used nowadays. It isn't too weird for a girl to use "dude" as long as you are relaxed about it. I can't speak for the rest of the world, but as far as the southern region of the US goes, if a girl calls a guy "dude" it is a casual way of affirming that they are just friends and she is not romantically interested in him, and she'll probably give him a side-hug after that. So if you're interested in an American guy, maybe don't call him dude, otherwise have it bruhhh.

Girls can use dude. We say it to both boys and girls.

Hello! "Dude" is a very informal word and it is perfectly acceptable for you to use it! Many non-native speakers tend to say something along the lines of "Hey guy" when they mean to be informal, however "Hey dude" would be more natural and correct. Enjoy, and if you have any more questions please contact me! Trenton

Ok, thank you for telling me this, I'll be careful when using some informal words with British