1st Mac (Snow Leopard)...not sure how to back up

Question

Just got my first Mac (iMac) with Snow Leopard; purchased from a friend who upgraded systems. I am EXCITED and want to upgrade to Mountain Lion but worried on doing something wrong; still learning the ropes if you know what I mean. I have an external (Lacie Rugged), and have tried the TM feature to backup. It backed-up without issues but this was several weeks ago. Since, Mountain Lion has released and I want it! So, I think I need to back-up the HD again because in these last several weeks I have been learning and tweaking the iMac to my personal setting and needs. It is slightly different from the original back-up. I do have all disc's in case something happens but I am trying to avoid a reformat. I will however if I absolutely have to.
My questions are:
1) will backing-up the system again over-write the previous backup file on the Lacie external? . . . or simply create a who;e new file in which I can then delete the first one?
2) will I need to delete the previous back-up file first (on the Lacie) and proceed with a new (second) back-up?
3) if the back-up is done with the Snow Leopard OS X, will it reinstall on the system should an issue occur if upgraded with the newly released Mountain Lion?
4) will reformat be a best option to simply start fresh?
The system runs flawless but has my friends name on the home (house icon) account. I am aware that to change this name, I think I need to create a new account. I am not sure however exactly how to do this or if I can delete his after I do; remember...1st Mac. (still learning). I did manage to change the name of the computer to mine. 
5) do files need to be transferred from his old (name; house icon) account to my newly created account in order to avoid issues if I do create this new account? if so, which specific files or all of them?
Ideally, I would like ONLY my info on this system but it's not a major concern for me since I know who I bought it from and that all previous owner settings were wiped clean: well almost.  I have read about prepping the system for the ML upgrade and verified and repaired the disk so that is done. I just need to make sure that a back-up is done correctly and if something should go wrong...I can get all the files back up and running even with the newly upgraded Mountain Lion OS X. I hope I am making sense here.
Anyway, I thought someone could give me a step by step break down on what exactly I need to do to get this done? ... and about the new account creation and how this exactly is done the RIGHT way.
Thanks

Answer 1

Well first you need to check to make sure your Mac meets the 10.8 specifications.
https://en.wikipedia.org/wiki/OS_X_Mountain_Lion
If you don't knwo what your Mac is, use the free MacTracker and then the Apple Menu > About this Mac > More informationa nd look for a model number like MacBook Pro 3,1 and use MacTracker search feature.
Next your going to have to make a copy of your existing data to a external storage drive, not TimeMachine which you currently have.
Most commonly used backup methods
When you upgrade to 10.8, your going to have a different machine anyway, so your going to be setting it up all over again. It doesn't make too much sense to restore from TM for just a few files as it takes a long time and copies over stuff from the previous owner. So it's faster and safer to just import your files manually when 10.8 is installed.
If your Mac will run 10.8, you have to realize that likely software you currently have (and is not yours) will no longer work or be there after this proceedure, it's to make sure it's all you and not someone elses.
First follow this method to erase and install 10.6 again, it's so you start clean and there are no conflicts/problems or issues  as your AppleID is used to download 10.8 and any app store software is copy protected.
This way your updating 10.6.8 (Software Updated) to 10.8 on a "Fresh install" which is better than trying a upgrade.
How to erase and install Snow Leopard 10.6
Make sure your iLife package is installed (look for iPhoto and if there it is) just use the bunded app installer if you have the grey/black 10.6 disks. White 10.6.3 disks don't have iLife, so you'll be buying it from the AppStore again later with your AppleID so you can get updates for it.
Next log into App Store and download 10.8 and upgrade, you do need a fast, reliable internet connection as it's large download.