How to convert .p12 to .crt file?

Try with given command

openssl pkcs12 -in filename.p12 -clcerts -nokeys -out filename.crt

You tagged 'keytool'. If you mean Java keytool, which is not the only one, it can do this:

    keytool -keystore in.p12 -storetype pkcs12 -exportcert -file out.crt -rfc -alias $name
    # for java9 up omit -storetype pkcs12 -- it's now default
    # -rfc gives PEM form; omit for DER form
    # can omit -alias $name if 'friendlyname' is mykey -- 
    # but that's likely only for stores created _with_ keytool 
    # because other tools and users mostly don't use that name

(but personally I'd use openssl as in crack_it's answer).