From the Feed page on any platform, just click in the share box and start composing your post.
You can include text, images, video, links to active content that will be embedded like YouTube or Vimeo URLs and even run polls.
The share box is found at the top of your Feed, just click in it and it will expand to look like the one above.
Check out the settings button too, it allows you to control notifications and other cool things like pinning your post to the top of peoples feeds.
Use the buttons in the top-right to choose which type of post you want to send and select who you want to share it with as below. You can post to whole team or to one individual (or any combination of those).