You have a domain registered through Google Domains and have generated an SSL certificate from them also?
How do I generate the certificate?
Netlify will automatically generate an SSL certificate for your custom domain using Let’s Encrypt, you don’t need your own.
So How do add it cause I am new to this
Once you’ve added your domain and it Netlify’s system has verified it is pointing to Netlify, the system will automatically generate an SSL certificate. You don’t need to enter any certificate details. You only need the details you highlighted in your original post if you have generated your own certificate using another service, or purchased one from a Certificate Authority.
How do I add DNS verification?and what should I set it to?
If you can share your domain name, we can make a more educated guess.