The Indivisible Guide
Indivisible (https://www.indivisible.org/) is great resource even if you aren’t active in an Indivisible group locally. Each Saturday the national team puts out a list of weekly priorities, many of which involve calls to Congress. Just go to their main page and scroll down slightly to this:
Each section on the left will have a corresponding action for the week. For actions involving a call to Congress, Indivisible almost always provides sample scripts as well as all of the background information on a particularly bill. The website has a ton of information under the Resources tab across a number of topics (e.g., Immigration, Tax Policy, etc.) as well as the Guide, which is the document that started the resistance.