hi,

see to get the primary knowledge about network and network security you should first start with youtube videos which are free. after that I would suggest you to chose between udemy or edx which is best course for network security from basic to advance level. however CISCO , one of the renowned company has ccna course which you may apply for primary level.

https://www.cisco.com/c/dam/en_us/training-events/certifications/security-infographic.pdf

hope it would help.