All you need do is insert the flash drive, restart the computer and after the chime press and hold down the OPTION key until the boot manager appears. If you're lucky the flash drive should appear as a choice. Select it and click on the downward pointing arrow key.
Now, normally you cannot boot a Mac from a foreign OS via a flash drive. You may find that despite what you've done you still cannot startup the Linux live CD from the flash drive. You will have to use an optical drive and boot from the Live CD. The problem is that the OS X bootloader simply will not recognize a foreign OS on a flash drive. Since your optical drive is not working you'll either need to have it replaced or buy an external one you can use.
Now, you may be able to get some different results working from Windows, but I've never installed Linux from Windows, and Windows is even worse about booting from any external device including flash drives. But, you may be able to get Ubuntu installed from Windows using Wubi which is included on the Live CD.
BTW, I'm 68. The first PCs appeared in the '70s. If you were in your 60s then you must be over 100 today. I don't know that I was being unfair, but if you've made it to 100 you deserve an applause.
In any event you still need to put some research time into what you plan to do before you try to do it. You're putting the cart before the horse. And that's a sure-fire plan for screwing up your computer no matter how old you may be.