how to download anything from commandline in fedora

You can look at wget or curl

can you give me a example say i want to download adobe flash player for my system

Do you want to install packages or download a file?

If you want to install packages you would use the package manager for your system (yum, in your case). An example would be something like:

$ sudo yum install inkscape

If all you want to do is download a particular file then you use wget with a specific address:

$ wget http://www.google.com 

Suppose I want to download the you tube video how to do it

If you want to play youtube videos in the browser, you need to install flash-plugin.

To download youtube videos, you can use the youtube-dl program. As so:

$ sudo yum install youtube-dl

which is a command-line program to download youtube videos. See docs here.

sudo: unable to mkdir /var/db/sudo/ka: Read-only file system

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.

ka is not in the sudoers file. This incident will be reported.

Error when giving command sudo yum install youtube-dl please hem me out of this

You do not have access to run sudo. You need to ask the system administrator to add you to the sudoers file. If you are the system administrator, you can also run that command as root.