Etiquette of leaving a job?

I've been in the same job since I graduated three years ago, and I've started looking for a change. Having never quit a job before, I am at a loss for how to go about it. Do I need to write a formal letter explaining why I'm leaving? Do I give it to my boss, or to HR? Should I start hinting now... show more I've been in the same job since I graduated three years ago, and I've started looking for a change. Having never quit a job before, I am at a loss for how to go about it.

Do I need to write a formal letter explaining why I'm leaving? Do I give it to my boss, or to HR?

Should I start hinting now that i'm thinking about leaving, so I won't take them by surprise? Or would it be rude to make it known that I'm tired of this job, and then stick around for potentially several months?

What else should I be doing that I haven't thought of?

(BTW, there's nothing *wrong* with the current job. It's just getting stagnant and at entry level I know I should be learning a greater variety of skills.)
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