Google Chrome is not an OS but an internet browser application. To find out what OS is running on your Mac go to About This Mac, click the Apple logo top left of screen. It will also tell you the model year.
When new the Mac would have come with two installation discs, both grey in colour, do you have them.
Mac OS X Snow Leopard is still for sale from the Apple Online Store (not the App Store) here is a link to the UK store,
https://www.apple.com/uk/shop/product/MC573Z/A/mac-os-x-106-snow-leopardif you are in another country just replace uk in the address field with your countries abbreviation.
If your MacBook is from 2007 you can probably upgrade it to Mac OS X El Capitan from here Upgrade to OS X El Capitan - Apple Support, or if it is from 2009 you may be able to upgrade to macOS Sierra through the App Store.
Whatever you decide to do you really need to make a back up on an external disc.