Ive bought In Design , my computer i Ma Os High Sierra 2017. How do I upgrade it?

In addition to @Barb Binder's suggestions:

The oldest macOS version officially supported by InDesign is macOS 11 Big Sur. I think you should still be able to install it on your 2017 iMac (not 100% sure, you'll need to look up your specific Mac specs). You may be able to run InDesign even on earlier versions (but you might as well not).

To upgrade the RAM, you'll need to buy the desired amount of RAM. Just find a few websites that sell RAM at reasonable prices and make sure to get RAM suitable for the exact model of your iMac. You can find instructions for RAM installation online. It's not much fun to install RAM into a computer that's combined with the monitor, but you'll get there. You can also take your Mac to a service place (Apple or otherwise) where this can be done for you for a fee.

We can help you with InDesign questions here, but the Apple forum is better suited to your iMac-specific questions: https://discussions.apple.com/welcome. 

Linking to these InDesign specs may help those who volunteer on the Apple forums help you more efficiently:
https://helpx.adobe.com/indesign/system-requirements.html

~Barb