How difficult is to convert normal HTML site to Drupal.

Drupal can take plain text as well as full HTML as page input.
Good luck with your Drupal project.

Drupal has a variety of modules to assist with importing previous data. One such module is HTML Import module (this module is not upgraded for Drupal 6 yet, so if you go this route, you would need to begin the site as Drupal 5, import your content, then upgrade the site to Drupal 6). This may only worth doing if you have a large amount of pages to process and don't want to do them manually (in any case I'd advise you to still check the results of every page manually, unless its a large number of pages, as you never know if an automated tool messed up somewhere).

Then on to the conversion of your website layout to Drupal... I've written a guide that I think may help: Convert any website layout or template into a Drupal theme - easily!
Also here's a video lesson that covers the same topic: Static Theme Conversion

I also have a variety of guides and help on my site in case you'd like to read... the one I most highly encourage is Creating a CCK and Views powered Drupal site

If you decide you need to hire help, I'm available.

I would advise converting by hand. If you're just starting out in Drupal 5 you may want to grab an existing theme to see what "guts" of Drupal are needed - the little PHP snippets and variables Drupal will be looking for. I suggest Garland since it comes with Drupal.

Open /themes/garland/page.tpl.php and look at the PHP parts in it to see examples of the guts. Hopefully if you have at least some HTML knowledge you will see that this file is just an HTML template with a few PHP snippets here and there. To get the basics started, you only need to take your existing HTML and put it into a page.tpl.php file inside your own theme folder -- for example if your theme is called "mytheme" your page.tpl file will be here: /themes/mytheme/page.tpl.php.

Once you have your HTML in this file you need to replace what would be your content areas (regions) with the PHP snippets. Examples: Drupal uses the $content variable where it will place content, it uses $sidebar_left for the left column ( the left column is a "region"), etc. You can actually just grab these PHP variables and replace your appropriate HTML areas. This will get you in the ballpark.

Then in the admin area, "administer > site building > blocks" click your theme name then tell Drupal what you want to show up in these variable areas -- you will not have to tell it that you want content in the "content region" but you will, for example, have to tell it what other regions should contain. For example if you had ads showing in the left column of your previous static web site but that has now been replaced in your template by $sidebar_left variable, you will need to create a block (name it something like "ads"), paste your former ad content into this new block, then for your new theme (called "mytheme"), tell Drupal you want the new "ads" block to show up in the "left sidebar" in the dropdown for this theme.

_{Way Cool Web Design | Theming Drupal 5}

I started using drupal to migrate the HTML site. I have thousands of HTML file in my site. Any automation process is exists for importing HTML content to drupal.

Can you please provide me information on using Import HTML module?