I need to create a workflow that will send out a task reminder on the 1st Monday of every month. How can I do this?
you could schedule a workflow on the 1st day of every month and inside find out what the current day of the week is and pause until the following Monday?
You will need to create a site workflow and then schedule it to run on the 1st of the month. Go to the gear in the upper right, go to Nintex Workflow, then Create Site Workflow. Once you've created your notification and published your flow, go back to the gear, and Nintex Workflow, then Schedule Site Workflows.
Note that this flow will be independent of any list, so if you need to gather list data you'll have to query for it.