How does one Copy, Paste and Tab using AppleScript?
Its hard to know exactly what you are trying to accomplish but some improvements were introduced into the code below, with comments added in the code.  Several points:


Note that depending on how fast/slow your system is responding you may need small delays after every keystroke or action.  You'll have to figure the minimum time empirically.
Any time you "activate" or launch Safari, Excel or any application within the script there will be a delay until the app is up and running.  To deal with this,  you will need these apps running prior to running the script, or you need to introduce long enough delays into your script, or better yet you need to add code that pauses the script until the app is launched.
When you open Safari and Excel you are not specifying any specific window, file or URL  to target for the subsequent keystrokes. So as written the script will default to the front window/tab of each app. I guess that's OK if what you are targeting is the front window. If no open window exists then the script will fail.  So for example, to get around this I added code to open a new excel file.
As currently constructed, the script copies from the front window in Safari, then eventually comes back to the very same window to execute the tab - paste - tab x 8. One has to assume that was the intention?  
As currently constructed the default is to paste to cell A1 of Excel.  Then you tell Excel to move to once cell to the left, but there is no cell to the left of A1. 
Much of the code is written for GUI scripting.  Where possible its better to use Applescript commands to accomplish the desired activity.  I'll just leave it like that for now.  You probably have enough on your mind learning the basics right now. 

tell application "Safari" to activate

tell application "System Events"
    tell process "Safari"
    keystroke "c" using command down
    delay 0.3
    end tell
end tell


tell application "Microsoft Excel" to activate
    delay 5 -- time required to launched Excel if not already open

tell application "System Events"
    tell process "Microsoft Excel"
    keystroke "n" using command down -- open new excel page
    delay 0.5 -- wait for new excel page to open
    keystroke "v" using command down
    delay 0.2 -- wait for key stroke entry
    key code 123
    delay 0.2 -- wait for key stroke entry
    keystroke "c" using command down
    delay 0.2 -- wait for key stroke entry
end tell

end tell


tell application "Safari" to activate

tell application "System Events"
    tell process "Safari"
    keystroke tab
    delay 0.2 -- wait for key stroke entry
    keystroke "v" using command down
    delay 0.2 -- wait for key stroke entry

    repeat 8 times
        keystroke tab
        delay 0.2 -- wait for key stroke entry
    end repeat

end tell

end tell