Having written the script, you can invoke it by sh scriptname,  or alternatively bash scriptname. (Not recommended is using sh <scriptname>, since this effectively disables reading from stdin within the script.) Much more convenient is to make the script itself directly executable with a chmod.
chmod 555 scriptname (gives everyone read/execute permission) 
chmod +rx scriptname (gives everyone read/execute permission)
chmod u+rx scriptname (gives only the script owner read/execute permission)
Having made the script executable, you may now test it by ./scriptname.  If it begins with a "sha-bang" line, invoking the script calls the correct command interpreter to run it.
As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root, of course), to make the script available to yourself and all other users as a system-wide executable. The script could then be invoked by simply typing scriptname [ENTER] from the command line.
Caution: invoking a Bash script by sh scriptname turns off Bash-specific extensions, and the script may therefore fail to execute.
A script needs read, as well as execute permission for it to run, since the shell needs to be able to read it.
Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where scriptname is located, why doesn't this work? This fails because, for security reasons, the current directory, "." is not included in a user's $PATH. It is therefore necessary to explicitly invoke the script in the current directory with a ./scriptname.