So you have an HTML template and you wish to convert into into a WordPress template?
Long story short...
Look in the /wp-content/themes/THEMENAME folder and modify the theme of your choice to include your new HTML markup. You can copy an existing theme folder and then modify 5the contents to create a new (separate) theme but this required a bit more work to successfully complete.
I haven't found the WP codex to be terribly easy to follow in my experiences with it, but there are many good tutorials out there that walk you through the steps of converting HTML to WP.
If you aren't comfortable with the way WP works, you still may have a hard time getting it converted successfully.
You may want to check out:
This is a online tool that converts your HTML/CSS template into a WP theme. I think it costs like $10 to do it, but it lets you preview how the template looks before you pay for it to make sure you like the way it converts.