Hi, small potato, welcome to the forum (fáilte romhat ar an fóram).
One the site's home page, click on Forum. Scroll down the page to find links to the four separate forum categories, General, Buntús Cainte, Irish Books & CDs, Irish Grammar. Click on the link for the forum that best matches what you want to write about for the occasion. Once in that forum, you will see a link near the top of the forum page to "Write a New Post". Click on that link and a new entry page will open up. Write your item, then click on the Post button at the bottom.
I had to hunt around a while myself when I first started in this forum, so I understand the frustration with finding this feature, but it is designed to keep the posts segregated into the four categories.
Dale D