I have a new macbook air, how do I block pop ups ?

Safari > Preferences... > Security > Block pop-up windows

It's not completely effective.

Get a respected ad blocker extension for your browser. I personally use Chrome as my browser and use the following extensions in various combinations, depending on my needs, to increase security and stow aggressing advertising and other malicious web content:

1) EFF HTTPS Everywhere

2) EFF Privacy Badger

3) uBlock Origin

Make sure that you ONLY download browser extensions from official sources such as Apple (for Safari), or the Chrome Web Store (for Chrome). In addition, do not use any extensions that you do not absolutely need.

Lastly, in addition to those browser extensions, you should look in to "OpenDNS Home" (https://www.opendns.com/home-internet-security/). It's a free DNS service that automatically blocks many malicious domains.

You should use the DNS servers provided by your ISP, in most cases.

If you are being inundated with popup windows while using Safari even on this site (for example) you may have other problems. Describe what you're experiencing.