I’m happy I got a new job, but I dread telling my boss. How do I do that?
First I recommend that you don’t delay and don’t tell others at work until you tell him or her – tell him/her first and privately.
Unless this is completely not true, say something to the effect of that it was a hard decision and mention something you’ve really enjoyed about your job, or learned, or benefitted from. Make sure you express your appreciation. For example, “It wasn’t an easy decision because I’ve really enjoyed working with the team/you’ve been a great boss/I’ve learned an incredible amount/etc, and I so appreciate the opportunity and my time here.”
It is important to give at least 2 weeks notice (more if you can) and tell him/her you want to make the transition as smooth as possible. Ask how you can do that – be prepared to go the extra mile and offer to document processes, train the next person, etc. And very importantly, make sure to go all out the remainder of your time there – be fully engaged and doing your very best.