I don't know how to rank for my keywords

In order to rank your category pages on desired keywords, you need to decide on variety of keywords you wish to increase rankings for, and to ensure your category page has snippets of articles shown on that page - and including those desired keywords. So, when you enable, say, 10 snippets with 100 words each, your category page will have 1000 words of content that eventually can be worth promoting. Since the content is getting updated (because new articles come in, and new snippet is then created each time), it is quite good signal for the search engines' bots to often visit your site, and rank it naturally over time (updated content is one very important ranking factor).

But, there is a catch. If you accept other people's content, then you can't control what keywords they will use and where they will put them, so your snippets may be constructed out of irrelevant parts of published articles.

In short, you need to ensure only quality content that is human moderated, is going to be published in your directory. Then figure out how only keyword relevant parts of articles can be gathered on each category page.

Then start to promote your category URLs by getting some high quality link building services available in this forum (check the one provided in my signature, too)