how do I clear the start up drive to download the new operating system

You should not have to clear the startup drive at all. Provided you have enough space on it to download the installer from the App Store, the installation files will download and then you simply need to follow the on-screen instructions to apply it. If there is a problem about there not being enough space, then you will need to remove some of your files to make room for this. For now, in the Finder press Shift-Command-C to reveal the hard drives on your system, and then select your current boot drive (the one likely called "Macintosh HD," unless you have renamed it), and press Command-i to get information on it. This will have fields that show how much space is used on the drive, and how much is free. If you are unsure whether or not you are low on space, reply here with these values and folks here can further instruct on what to do.

Did the installer say you needed to free up space on your startup disk? If so, you need to remove some of you documents, movies, photos, etc. to make space available for the OS. You can either move them to an external drive or delete them if you don't want them anymore.

Here are some general tips to keep your Mac's hard drive trim and slim as possible

You should never, EVER let a computer hard drive get completely full, EVER!

With Macs and OS X, you shouldn't let the hard drive get below 15 GBs or less of free data space.

If it does, it's time for some hard drive housecleaning.

Follow some of my tips for cleaning out, deleting and archiving data from your Mac's internal hard drive.

Have you emptied your Mac's Trash icon in the Dock?

If you use iPhoto or Aperture, both have its own trash that needs to be emptied, also.

If you store images in other locations other than iPhoto, then you will have to weed through these to determine what to archive and what to delete.

If you are an iMovie/ Final Cut user, both apps have their own individual Trash location that needs to be emptied, too!

If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!

Delete any old or no longer needed emails and/or archive to disc, flash drives or external hard drive, older emails you want to save.

Look through your other Mailboxes and other Mail categories to see If there is other mail you can archive and/or delete.

STAY AWAY FROM DELETING ANY FILES FROM OS X SYSTEM FOLDER!

Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.

Again, archive to disc, flash drives, ext. hard drives or delete any old documents you no longer use or immediately need.

Look in your Applications folder, if you have applications you haven't used in a long time, if the app doesn't have a dedicated uninstaller, then you can simply drag it into the OS X Trash icon. IF the application has an uninstaller app, then use it to completely delete the app from your Mac.

To find other large files, download an app called Omni Disk Sweeper.

http://www.omnigroup.com/more

Also, Find Any File

http://apps.tempel.org/FindAnyFile/

Download an app called OnyX for your version of OS X.

http://www.titanium.free.fr/downloadonyx.php

When you install and launch it, let it do its initial automatic tests, then go to the cleaning and maintenance tabs and run the maintenance tabs that let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.

Typically, iTunes and iPhoto libraries are the biggest users of HD space.

move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.

If you have any other large folders of personal data or projects, these should be archived or moved, also, to the optical discs, flash drives or external hard drive and then either archived to disc and/or deleted off your internal hard drive.

Moving iTunes library

http://support.apple.com/kb/HT1449

Moving iPhoto library

http://support.apple.com/kb/PH2506

Moving iMovie projects folder

http://support.apple.com/kb/ph2289

Good Luck!