Let your visitors know whats important
When website visitors (and search engines) arrive on your page, they are going to immediately try to assess the most important topics on the page, and if they match up with what they are looking for, they will continue reading your text. The best advice I can give you for writing your content is to use your header tags properly. Most web editors will let you enter these header tags, but you may need to switch over to your source code to add the tags properly.
Determine the what topics your page covers
At the top of your page, make sure your primary topic is set in an h1 tag, it is ok to adjust your css to compensate for the larger size it will make that text by default. As you write the rest of your page, break up your sub-categories by applying h2 and h3 tags to them so that those titles describe the content visually and make it easy for the site visitor to digest the different topics that you are discussing. Of course, your page content should all reflect your primary topic in some way, putting completely different topics on the same page makes it difficult for both visitors and search engines to determine what you are relevant to.