Need help to create final art

Before you do this, it's a good practice to do a visual check on screen (soft proof) to see what difference between your RGB and what you will get in CMYK based on the profile you are converting to:  Under View: set your color profile (e.g. GRACOL or whatever), then toggle the Proof Colors to go from RGB <> CMYK. This is particlarly useful to see where out of gamut colours (like bright blues and intense pinks) will land in CMYK.

No need to change your files (in fact don't!), but if this particular vendor will not accept anything but CMYK (dark ages thinking, fwiw). you can convert your colours to CMYK on exporting your PDF. Ideally, this vendor should provide you with a color profile specific to their press to use, but failing that, use what is currently set as your CMYK profile in your Color settings (if yours is set at the default when installed it may be US Web Coated (which is pretty old school)... better yet, use a generally-accepted modern one, e.g GRACOL as in my example.

Change the section of Output from "No Color Conversion" to "Convert to Destination (Preserve Numbers)". This will convert all your documents RGB values to the closest possible value into CMYK values. (*The Preserve Numbers is added to make sure any colors you already have in your document that are already CMYK do not get changed).