If i enter Creation Kit , change a little bit of my MOD and then hit "save" will it automatically go to Steam Workshop ? So people can update their copy of my Mod ?

If not, what i need to do ?

No, it won't. Not unless you submit it (File>Upload Plugin and Archive to Steam)

If you submit it today, and choose to work on it tonight, it won't resubmit your work until you choose to Upload it again. One thing though, don't "Subscribe" to your own mod, as I think doing so would repeatedly wipe out any changes you've made to it anytime you load up Skyrim and it synchs with your uploaded version.

Thanks !