hi I just bought new imac and its taking all 8gb of RAM just to browse safari. How much RAM imac use just for being switch on? while writing to you now i have only 16 MB free.

You are applying old standards to a modern Mac. As my colleagues have stated, Apple changed the way RAM is allocated around Mac OS 10.9 or 10.10. The old memory metrics of RAM "free" and "used' are obsolete and misleading.

The new metrics are Memory Pressure and Swap Used. This is from Activitty Monitor's "Memory" tab:

If Pressure is in the green and swaps are under about 250MB, you are fine.

The labels are explained in this Apple Support article:

How to use Activity Monitor on your Mac - Apple Support

However, Activity Monitor reporting that Safari is using that much suggest that there is third-party software or conflicting Safari extensions at play. If you cannot return your Mac we need to eliminate know RAM wasters.

Have you installed:

1) anti-virus software

2) so-called "cleaning/tune-up" software?

3) torrent apps

4) are you using an online backup scheme like Carbonite, OneDrive, GoogleDrive, etc?

5) Do you have more than one ad blocker installed in Safari?

Although 16BG RAM is better than 8GB, I have seen reports from recent iMacs with 8GB RAM that are not starved. That is what leads me to suspect some form of software interference/conflict.

8GB of RAM is considered the bare minimum. If you bought a 27" it is was to upgrade the RAM, I'd suggest adding an additional 16 GB of RAM to get to 24GB. If you have a 27" and want to do this, PLEASE only buy RAM from Crucial.com or macsales.com. Why, easy these iMacs are very sensitive to using quality RAM and both vendors sell high quality products at reasonable prices and provide excellent service. In short, there is no reason not to use them.

However, if you bought a 21.5" iMac then return it ASAP and buy one with 16GB or more of RAM. 21.5" iMacs are not user upgradeable so you got what you got! If you bought directly from Apple then you have 14 days to return for ANY reason. If bought from some other reseller then you must work within that resellers sale and return policies are.

Macs will delegate as much RAM as possible to a running program. However when you switch to another program it will keep your browser open with as little RAM as necessary and delegate the most RAM to the new open program.

While 8GB of RAM is a bare minimum, it may not be a problem depending on which apps you run. For instance, you won't be able to do much video or music editing. If you can upgrade the RAM, do so,

Just to add to what Allan stated, I have among other Macs, a MacBook and a MacBook Air with 8GB RAM and have no issues running more than one app at a time on them. Safari should not be using 8GB unless. you have dozens of tabs open all running videos etc.