This is directly from the Instagram website:
How do I post a video on Instagram?
To upload a video or record a new one, first tap at the bottom of the screen:
To upload a video from your phone’s library, tap Library (iPhone) or Gallery (Android) at the bottom of the screen and select the video you’d like to share.
To record a video, tap Video at the bottom of the screen. Tap and hold to start recording, and lift your finger to stop. You can tap and hold again to record multiple video clips and tap to switch between cameras.
Note: You can’t take or upload videos from a desktop computer. MORE INFO HERE:
But that is only the beginning…
Here are more resources to help you with Instagram Video: