Hi Trev the Traveller. Welcome. On the opening page, go to the section you want to post in ie General Forum, Techies Corner or What's Cooking and you will notice on the right hand side, 'Start a New Topic'. You use this button if you want to start a new topic. Once you click it, it will open a 'posting page'. Write what youy want to say and then scroll down until you see 'Submit post'. Click this. Your post will then go to the right section and you will see a little tag against it which will mean you started that post.
If you only want to add something to a thread of conversation in a topic, the quick way is to start writing your answer in the space at the bottom of the posts, then hit 'post quick reply'. If you want to change something you've written you have a short time to go back and hit 'edit post' and change it before hitting 'submit post' in the bottom right corner. Hope that helps. You'll soon get used to it and others will have other ways of doing it.