With the whole team... Select Teams, pick a team and channel, write your message and click Send. 

With a person or group... Click New chat, type the name of the person or group in the To field, write your message, and click Send. 

A team is a collection of people, conversations, files, and tools--- all in one place. 

A channel is a discussion in a team, dedicated to department, project or topic. 

Select Teams and choose a team. Pick a channel to explore Posts, Files and other tabs. 

Click Meet now under the area where you type a message to start a meeting in a channel. (if you click Reply, then Meet now, the meeting is based on that conversation.) Enter a name for the meeting, then start inviting people. 

Channel posts are organized by date and then threaded. 

Find the thread you want to reply to, then click Reply. 

Add your thoughts and click Send.

To get someone's attention, type @, then their name (or pick them from the list that appears). 

Type @team to message everyone in a team or @channel to notify everyone who favorited that channel. 

Click Sticker under the box where you type your message, then pick a meme or sticker from one of the categories. There are also buttons for adding an emoji or GIF. 

Click Attach under the box where you type messages, select the file location and then the file you want.

Depending on the location of the file, you'll get options for uploading a copy, sharing a link or other ways to share.

Type a phrase in the command box at the top of the app and press Enter. 

Then select the Messages, People, or Files tab. 

Select at item or use the filters to refine your results. 

Click Help on the left, then choose Topics, Training, or What's new. 

In help, browse the tabs or search for a topic you're interested in.