how do I clean my computer before downloading windows 10?

? can you explain what you understand under "cleaning before installing windows 10" ?
Do you want to perform a clean install of windows 10?
 
There are 2 ways of getting windows 10 to install on your system.
If your operating system now is windows 7SP1 or windows 8.1 you could allow your system to do an upgrade thru the GWX
shortcut that should be available on your task bar ( look for the windows 10 icon there ). Then let it upgrade and then once windows 10 is install download the media creation tool from microsoft and create a USB thumbdrive with the install files from windows 10 
Then when that thumb drive 's been created you can wipe your system clean with a tool like killdisk or DBAN ( might take a few hrs  depending on the size of your hard drive ) and then boot from the thumb drive & do a clean install. This way if you do it right you will not be requested to provide a product key.
 
Secondly you could do it faster but then you will be required to use a windows 7 or 8.1 product key to activate your windows 10 clean install from the thumb drive...
 
I prefer the first way as then during the upgrade your activation info will be stored somewhere on a chip or on the microsoft server. 
 
That's it in a nutshell. If you require more details then let me know and i'll provide you with more info via a PM or email depending on your choice. 
 
Be aware that you do need a thumb drive with a capacity of at least 8Gb or else you will not be able to get all the files to store on the thumb drive.. The second way described will only work if your download copy of windows 10 is at least 
build 1511 version 105386 ( the november 2015 upgrade from windows 10 ).
 
And be sure that you do have your product key from Secure Anywhere available so you can re-install Secure anywhere after the upgrade. 

Hi ESESES
 
Welcome to the Community Forums.
 
Let's keep it simple; (i) run a good cleaner such as CCleaner or the like to clear out the garbage, (ii) defrag your hard drive or optimise you SSD to make sure that all your free space is appropriately positioned, etc.
 
That's it...nothing more is required. I did the above and WIndows 10 upgraded fine on the two systems I have done this for.
 
Regards, Baldrick