The Slack Web API is an interface for querying information from and enacting change in a Slack workspace.
Because we strongly recommend you do not use legacy custom integrations anymore, you should instead use the Web API with Slack apps. Our guide to the Web API will walk you through the process of using it in a Slack app.
If you were using a legacy token to make calls with the Web API, you'll need to generate a new one for your new Slack app. If you chose to build a workspace app, you can install it to your own workspace in a couple of clicks. Then prepare it for distribution by following our guide to using OAuth to generate workspace tokens. Traditional Slack apps can follow our guide to the OAuth flow.
Once it's installed, use the app's settings to progressively add the permission scopes you need for your testing.