When you talk to co-workers, you will eventually tell them that you are leaving. Here are some general sentences.

"Next Friday is my last day."
"I already put in my two week notice."
"I'm leaving the company."

"I accepted a job offer from ABC Company. This Friday will be my last day here."

"I'm leaving this dump."
"I would've left a lot sooner but I had to get a job lined up first."
"I got screwed here so many times. I don't want to work here anymore. My last day is this Friday."

"It was great working with you. I wish you the best on this project."
"I had a wonderful time here. I wish the best for you."
"I really enjoyed working with you. I'll send you my personal email so keep in touch."

If someone else is leaving, then you can say something good like the example sentences below.

"I hope you enjoy your new job. Let me know if they have any other opening."
"Good luck in your new job. We'll keep your seat warm for you in case you want to return."
"Hey Jack, good luck with your new job and save me a seat."
"We'll miss you here. Good luck with your new career."