Convert document to CMYK

If the printer wants an all CMYK PDF, they’ll need to provide you with their press profile and that profile needs to be assigned to your document Edit>Assign Profiles. Even if you export with the default PDF/X-4 preset which leaves color unchanged, you’ll need the correct CMYK profile assigned to your document, because the CMYK profile handles the preview of your native ID CMYK swatches and is set as the PDF’s Output Intent, which handles the preview in Acrobat.

PDF/X-4 leaves transparency effects live (not flattened) and color unchanged. With placed RGB images you’ll need to turn on Overprint or Separation Preview to see how unprintable RGB colors will print. To get an all CMYK PDF/X-4 set your Output>Color Conversion to Convert to Destination and the Destination to Document CMYK.

PDF/X-1a flattens transparency and forces all process color into document CMYK.

In addition to what Derek said, "CMYK" is a device-dependent color space. So you must how the colors should "look", which is described by a ICC color profile. You should talk to your printer about the correct profile, so you can do the RGB -> CMYK conversion during the PDF export correct. If you don't know that you can stop here.

In theory there is a different profile for every paper-machine combination. But there are some industry-standards like PSO coated/uncoated v3 in Europa for offset printing. Not sure what the standards are outside of Europa (GRAcol? SWOP?)

Presumably you're sending them a PDF, select PDF/X-4, normally single pages, (not spreads), tick Crop Marks, and Use Document Bleed Settings – but ask the printers to confirm the spec, they should do the conversion.