The Keyword that is most relevant to your put H1 there. Like if you site is providiing Computer Accessories that your main keyword should be Computer Accessories and put h1 over there. The keyword related to that but having less importance than that you can put h2 and h3..h6.
well ... Google gives first importance to title than meta and H1 tag.. use your initial keywords which you are thinking to optimize in title and H1 tag..but it should look natural ...title should be less than 65 characters if you wanna create seo friendly title of specific page.
you can add like this in your webpage code <h1>your content</h2> you should use in your main article heading.it tell google this is more important than other content... you'd better view google seo guide...search in google..you can find it....there are more details...