HTML
The plain HTML way is to put it in a <form> wherein you specify the desired target URL in the action attribute.
<form action="http://google.com">
    <input type="submit" value="Go to Google">
</form>
Set if necessary CSS display: inline; on the form to keep it in the flow with the surrounding text.
CSS
If CSS is allowed, simply use an <a> which you style to look like a button using among others the appearance property (only IE support is currently (July 2015) still poor).
<a href="http://google.com" class="button">Go to Google</a>
a.button {
    -webkit-appearance: button;
    -moz-appearance: button;
    appearance: button;
    text-decoration: none;
    color: initial;
}
Or pick one of those many CSS libraries like Bootstrap.
JS
If JavaScript is allowed, set the window.location.href.
<input type="button" onclick="location.href='http://google.com';" value="Go to Google" />
Source