This can all be done in the settings in your Shopify admin. Navigate to settings > Shipping and delivery > Manage rates (found in the first section "Shipping).
From there you will be able to adjust how much you charge for shipping. Look at Shopify's comprehensive guide on shipping rates for more information.
Eunice here from Shopify, thanks for posting and welcome to the Community!
From your message, it sounds like you want free shipping to be your default shipping rate on all orders. If that's right there's an awesome help doc here that shows you how to set that up.
I did see in your message though that you've followed instructions and advice already and nothing is working. Please let me know what you've already tried so we can take a look at what's going on here.
I look forward to hearing from you!