Program opens automatically upon start up

I have a MacBook Pro Mid-2009. In January the operating system was upgraded to 10.10.2. This change necessitated an upgrade of Office for Mac to the 2011 version. This is the where the situation begins. On starting the computer M/S Excel opens and I see a new worksheet. I prefer to see my desktop. I came to Support Communities. I examined posted questions and 6 days prior Tracy posted something very close to my concern. A response from Matt Clifton included 3 possible actions to address Tracy's issue. Tracy's response stated problem was fixed, but did not indicate which action produced the desired result. Some of the terms/descriptions Matt used in his response to Tracy I did not understand. I was able to follow the second suggestion which was 1. Go to System Preferences, 2. click on Accounts, and 3. click on Log-in Items. What differed is that in my System Preferences, "Accounts", doesn't exist, instead it has "Internet Accounts" and there was nothing I saw indicating "Log-in Items"

One option offered used the term re-boot, I don't exactly know what that is. Can somebody elaborate on what it is and how one would go about a re-boot.

And another option used the term "right click", Apple doesn't us a two button mouse so there truly cannot be a right click. So what was Matt trying to say?

Or if you have other thoughts to address the unwanted opening of Excel with computer start up please contact me.

Thanks for your suggestions.