To search on the forum, click the magnifying glass icon in the upper right of the page, and then enter your search terms in the box that appears. By default, the search is limited to the forum/category that you’re viewing, but you can expand it to the entire site by deselecting the checkbox at the top of the results. The Discourse system primarily looks for your search terms in topics and their posts, but it can also find users and categories.
If you’re having trouble finding the post you want, use the Advanced Search page. There you can limit your search by user, category, tag, date, and other criteria.