Learn as much networking as you can, being IP, ARP, ICMP, routing, then TCP/UDP, then learn about the higher level protocols such as DNS, HTTP(S), etc. Understand what is actually happening "on the wire", so to speak. Tools such as Wireshark can be invaluable for understanding low level networking. Intercepting proxies such as ZAP, Burp, etc are great for understanding what is going on in HTTP(s) sessions.
Also look at things from the other end of the stack. Do some development of network apps, web sites, etc, etc. The best security folk I know have insight into both sides. Attack from "below", but also understand what is likely going on in the application as well.
Courses such as CEH, etc are VERY dependent on the quality of the trainer. If you are going to do a course, find out who will be giving the course, and research that person to see what their qualifications are, as well as any feedback on their abilities as a trainer are.