If you already have your css file defined, you should be able to add the new style for H1 at the end of the style sheet. A small basic example:
h1 {
font-family: "Courier New", Courier, mono;
font-size: 16px;
font-weight: bold;
}

This simply redefines the H1 html tag using the css sheet.

Then, all <H1> tags should default to the redefined H1 css style.

The other option is to create a custom class, such as .heading1 It would look similar only:

.heading1
{
font-family: "Courier New", Courier, mono;
font-size: 16px;
font-weight: bold;
}

Then in your html code, any place you want the style applied to the H1 tag, you should be able to use:

<H1 class="heading1">Text goes here</H1>

I am still relatively new to css, so I hope I am sending you in the right direction. You can test it on a sample html and css file to avoid modification to your entire site.