When you're chatting with one or more people in Teams, you can instantly share your desktop or screen. Just go to the chat controls in the upper-right corner and click Start sharing your screen .
Click a window to share that specific program and its content, or click Desktop to share everything on your screen.
The other participants in the chat will receive a notification asking them to accept your screen share. Once they do, they'll be able to see your screen and continue the chat.
Note: When you share your screen in a chat, you can't also turn that chat into a call. You need to end the sharing session and start a new call to be able to talk to the other people.