Assuming that you want to send a mass email to all of your ACTIVE members:
From you dashboard:
Click on membership status report on the right side of the screen, then you will see all the different statuses. Click on ACTIVE, and you will find the email icon next to each client of yours. But also an email icon at the header (highlighted) that will allow you to email that entire list a particular thing. This email icon will be located on several locations/reports across the system, that makes it easier to segment groups of people.
After you have found the list you want to email to, you can either select from the dropdown one of the form letter already built(welcome letter to send the member credentils), or create your own by clicking onManage form letters.