To like a photo or video, tap on the bottom left of the post. The person will receive a notification that you liked their post.
To unlike a post, just tap again.
To comment on a photo of video, tapon the bottom of the post, then tap Add a comment. Once you're done, hit Send. You can use your phone's built in emoji to comment as well.
You can reply to someone in a post by mentioning them, and they will receive a notification of your mention.