how do i set up icloud on my chromebook asus c300

If you mean that you want to use a mail app on the Chromebook for iCloud email, see: Mail server settings for iCloud email clients - Apple Support

Note that as described there you will need to use an app-specific password rather than your usual Apple ID password and that requires that you turn on two-factor authentication. https://support.apple.com/en-us/HT204397

Any other access to iCloud from a Chromebook would be by using the browser and going to www.icloud.com.