While it's certainly possible to craft a separate web page to do this, it sounds more like you're simply looking for a macro program of sorts to help you auto-fill in forms. There's plenty of software out there for that kind of thing (such as RoboForm and LastPass or even just every web browser's autofill features)
If your looking for something more home grown and programmable, a userscript sounds like it would be simple enough to write. Just create a javascript file named mytemplates.user.js and put something like this into it (solution to include jquery stolen from here):
// ==UserScript==
// @name Name your script here
// @namespace http://doesnthavetobearealwebsite.com/
// @include http://sitewhereyoufillinformsdomain.com/*
// @author Your name here
// @description Script which fills out our forms!
// ==/UserScript==
// a function that loads jQuery and calls a callback function when jQuery has finished loading
function addJQuery(callback) {
var script = document.createElement("script");
script.setAttribute("src", "http://code.jquery.com/jquery.min.js");
script.addEventListener('load', function() {
var script = document.createElement("script");
script.textContent = "(" + callback.toString() + ")();";
document.body.appendChild(script);
}, false);
document.body.appendChild(script);
}
// the guts of this userscript
function main() {
$('body').append($('<button>My Template</button>')).click(function(){
$('[name*=firstName]').val('Initial Value');
$('[name*=lastName]').val('Another template value');
});
}
// load jQuery and execute the main function
addJQuery(main);
This would add a button to the bottom of the page which, when clicked, would fill out the form using the code above.
Expand the main section out to include all your form elements.
In Chrome or Firefox (with the GreaseMonkey extension) just hit Ctrl+O to open the "open" dialog and then open the file. You'll be asked if you wish to install the extension.
Repeat each of the lines above for each query-able form element and you're good to go. If you don't know jQuery or JavaScript, go learn it!