Go to the main forum page of the subject that you are interested in. e.g. For CSS topics go here. Then at the top of the forum there is a big orange button that says "+Post New thread". Click that button and post your thread giving an appropriate thread title name and then a good description of your problem or question and provide as many details as possible on order to help us understand what you need.
When answering questions to existing threads then just use the quick reply box at the bottom.
There are a number of ways to navigate to the area you want.
You can use the big menubar at the top of the page and click the big orange "Forum" Link which will take you here.
Then just scroll down the page and all the areas are listed under their own headings. For example there is a main heading called "Design Your site" which has a number of sub forms all listed below. You can find the CSS forum listed under Web page design.
If you want to go directly to another forum then at the bottom of the page just under the pagination you will see a "quick links box" that will show the current forum name. Click the down arrow to open a list of all the forums that you can scroll down one at a time.