<form method="get" action="mvshayminget.html">
<span style="font-family: Arial, sans-serif; font-weight: bold;">Input a password to get Mystery Valley Shaymin:</span><br />
<span style="font-family: Arial, sans-serif;">Password:</span>
<input type="password" name="flowertown" maxlength="12" size="12" /><br />
<input type="submit" value="Submit" />
</form>
That's the proper form for your password field but you should know a couple of things. First, try to use relative links when possible (I did so for the action of the form since these two pages are likely in the same directory). You don't have to specify the entire URL when he pages are being called from the same site.
Also, while this will successfully submit and take you to mvshayminget.html, you'd need code on that page to process the password or you'll just wind up making a fancy link. Something like PHP, or at least JavaScript, on your target page will be required to check the password before displaying the appropriate content if you would actually like that functionality. If you go the route of JavaScript, do be aware that the password would be visible in the source code where as using PHP, it wouldn't be.
Just a couple of things to take note of as you start to learn :)