You can see a list of your SendGrid API keys via the "API Keys" section of your SendGrid account's "Settings" page. If you'd like to use an existing API key, you'll have to get this from whoever set up the key — SendGrid doesn't let you display API keys after they're created. If you want to create a new API key to use with Retool, you can press the "Create API Key" button to create a new key and view it. Store it somewhere safe because SendGrid won't show it to you again!
SendGrid > Settings > API Keys
Create a new resource in Retool, and select "SendGrid" as the type. Enter your API key in the "Access Token" field. Press "Save" when you're done.
Resources > Add > SendGrid
You can now select your newly-created SendGrid resource from the Resource dropdown when creating queries in your Retool apps. Please refer to the SendGrid API docs for details on each operation: https://sendgrid.com/docs/API_Reference/api_v3.html.