i recently bought an i mac i have not yet installed yoesmite os .i have 8gb memory and 1tb hard drive. how much memory will the new o s take up on my mac...will it make it slower? thanks

I have almost the same machine and it works great with 8 GB of RAM .

OS X Yosemite System requirements: 2 GB of Ram and at least 8 GB on your hard drive.

How to install OS X Yosemite on your Mac - Apple Support

You've gotten your terms mixed up. The machine loads everything that it uses into RAM (random access memory), leaving the original data alone until you save or delete something. To install the newer OS requires at least 8 GB of free DISK space, not memory. You'll have to identify the exact machine you bought and then check it's specs for the maximum amount RAM that you can install. AFAICT, Yosemite is as fast or faster than Mavericks.

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10.4), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2

THank you so new os is saved on ram.

No. The new OS is saved on the DISK. However, when you first startup (boot) the machine, all the required files needed to run the machine are copied into RAM and that's what the machine uses until you delete, save, or change something, then the files on DISK are changed. When you shutdown, the RAM is cleared and you're ready to go again.

Marinoboy67 wrote:

thank you very much. very helpful will prpbably have lots more questions just getting use to apple....thanks again

Since you're a newcomer to the Mac, see these:

Switching from Windows to Mac OS X,
Mac Basics—Tutorials on using a Mac,
Mac OS X keyboard shortcuts,
Apple How-To Guides for Mac OS X,
Anatomy of a Mac,
MacTips,
Switching to Mac Superguide, and
Switching to the Mac: The Missing Manual, Mavericks Edition.

Additionally, *Texas Mac Man* recommends:

Quick Assist,
Welcome to the Switch To A Mac Guides,
Take Control E-books, and
A guide for switching to a Mac.