There are two ways you can whiteboard in Teams: use Microsoft Whiteboard or Freehand by Invision.
Use Microsoft Whiteboard
Each Teams meeting has a dedicated whiteboard where participants have space to sketch together.
For detailed info about using Whiteboard in Teams, see Use Whiteboard in Microsoft Teams.
Use Freehand by Invision
Freehand is a whiteboarding tool from Invision that you can use in meetings to sketch with other participants.
To use Freehand in a meeting, click Share in your meeting controls, then click Freehand by Invision in the Whiteboard section. As the sharer, you'll need to sign in with your Invision account.
Then, you can begin sketching and sharing with participants.
Once you've shared a whiteboard, the meeting will have a whiteboard tab so participants can easily reference it later.
Note: This feature is only available if an admin has enabled it.