In "Communities" option select those you are interested in and press "Join". Now you can post comments and create new threads n this community.
If none of the communities suits you, create your own by pressing "New Community". In a drop-down window download the image and background, specify name, region and provide description.
Go to the community page and select "Write a Post". Specify topic, add images, videos or audios. Note, that you can create new topics only in communities you have already joined.