Do I need ccna for pentest?
I know it's about network administration but I was wondering if it will help me see things the way the sysadmin does.

All these questions about what a sysadmin would do... How about getting a sysadmin/network admin job in some corp for a few years?

You can get as many certs as you like, they will only teach you standard ways of doing things. Some real xp will teach you of tricks and methods you wouldn't find in books

@netikras thanks
Well network administration is so related to pentest.
Is there any forum you would recommend?

I 100% agree with the sentiment of @netikras answer. However if you are lucky enough to get CCNA for free, at least the first part, it's really great for getting a basic understanding of networking, that you won't get with just experience