In Photoshop, how do I save my work in CMYK? 300 ppi?

I'm trying to submit some of my images to a magazine, and the requirements are

300 ppi, 8 bits in CMYK as TIFF or JPEG

- How do I save it in 300 ppi?

- I tried to covert it to CMYK color from Image--mode but I lost a lot of colors! Is there a way that I can convert it to CMYK without losing any color?
 
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