You can write a new discussion topic to the forum if the teacher has added the forum to the course site (News forum is available only to the teachers). You can post the message by clicking Add a new topic. After this you are able to write the subject and the message (required). It is also important to notice the subscription option. There you can choose if you like to receive the messages via an email or not. In general you can always add an attachment to the message or utilize the other media buttons such as adding Kaltura videos by using Embed Kaltura Media button. When you have sent the message by clicking Post to forum you have 30 minutes to edit the post.