Is it better to post new content on the front page or create a separate category (like blog) where I will put my articles in? The site is commercial so I think it won't be good to post full text in the front. But I my front page is the most important and I want it to be ranked high. Should I mention only description or titles of my content on the front page with the link to the full text? Any advice?
The best way to add fresh content to your site IMO is to add new pages... not by changing existing ones. Each page you add is a chance to have a new <title>, new <h1>, new <h2>, new content, new keyword rich page name, etc. where you can target a new keyword phrase without chancing messing up your rankings for existing pages by changing their content. Your home page should be targeting your head terms (hardest to rank for terms/phrases). The deeper you link into your site, the more long tail your targeted keyword phrases should become. I'm a big fan of theme pyramids as a site architecture. It's based on this concept.