Posts are created from any of the feed views in the app. This means within any of the groups, or on the main feed page. To create a post, click on the large red + button on the screen. The post creation box appears on the top of the feed:
You can even choose which group to post to directly from the post creator by clicking the pencil icon next to the group name, then hitting the 'Post to group' on the top left of the post (this is the default for posts from the main feed):
Upload files and images that are relevant to the post, so that all of your data is linked together via a single post. You can even upload multiple images and the post view is a beautiful aggregation of the images:
Protip: We strongly suggest you #tag and @mention in your posts with key words as you create them. This will help you immensely over time as you want to look back quickly at related posts, or search through a labmates posts for specific data.