Sorry i am new to this field and learning.
We have ATEM 1 M/E Production Switcher with 3 cameras. If we would like to stream video live what else (soft/hardware) we will need and how can we accomplish this, any help would be appreciated.
Some of it depends on what service you are using to stream. Livestream.com for example supplies their own software to use with a video capture card. Others will work using the free Adobe Flash Media Live Encoder.
In both of those cases you will need to get a video capture card because the built-in USB 3 port requires using a very specific chipset on your computer which is hard to get ahold of these days.
If you have a thunderbolt equipped computer you can get an UltraStudio Mini Recorder for ~$145, or if you want to go completely computer-free you could get a box like the Teradek VidiU or the Matrox Monarch HD.