If this is the only redirect you'll be doing, the rewrite module is going to be more confusing than necessary if you're not already familiar with it. I recommend creating a Default site that loads when you go to the root URL (http://myServer/), with the following code in a file named default.asp:
<%
Response.Status="301 Moved Permanently"
Response.AddHeader "Location","http://myServer/sites/site1"
%>
That will do a permanent redirect. If you'd rather do a temporary redirect (so search engines will check there later), do a 302 redirect:
<%
Response.Redirect "http://myServer/sites/site1"
Response.End
%>
Or, if you'd like to still use IIS URL Rewrite, the regex should just check for an empty string:
^$