On the main forum page...
//www.agardenersforum.com/forum/ubbthreads.php/ubb/cfrm
...scroll down and find the forum that best fits your question.
If your question is about a house plant, you would click on the "House Plants" forum. If your question is about vegetable gardening, you would click on the "Fruits and Vegetables Gardening" forum...etc...etc...
...and there are subforums in the main forums, so if your question is about a tomato, you go to the vegetable forum and you will find a subforum there specifically for tomato questions.
When you click on the topic that best suits your question ("House Plants", "Plants and
flowers", "Shrubs and Trees", etc...) click the blue button that says
"New Topic".
That will take you to a new page where you type your question.
Make sure to put a title in the subject line.When you are finished, just hit the
"Submit" button and your message will post on the forum.