The default display style for a div is "block." This means that each new div will be under the prior one.
You can:
Override the flow style by using float as @Sarfraz suggests.
or
Change your html to use something other than divs for elements you want on the same line. I suggest that you just leave out the divs for the "last_name" field
<form action="/users" method="post"><div style="margin:0;padding:0">
<div>
<label for="username">First Name</label>
<input id="user_first_name" name="user[first_name]" size="30" type="text" />
<label for="name">Last Name</label>
<input id="user_last_name" name="user[last_name]" size="30" type="text" />
</div>
... rest is same