We'll need to know what software you use for your web server (apache, nginx, etc) to give you a proper url rewrite rule.
Another possibility is to place an index.html in the web root for domain.com that contains the following code (adjust as necessary):
Code:
<html>
<head>
<title>Your Page Title</title>
<meta http-equiv="REFRESH" content="0;url=http://subdomain.domain.com/">
</head>
<body>
Optional page text here.
</body>
</html>
This is basically just an html redirect, so when someone visits domain.com and their browser loads index.html, it will see the <meta> tag telling it to "refresh to url", or in other words, to redirect to another url.