Ability to create a flexible forms in variety of layouts is one of the key strengths of Agile Toolkit CSS framework. When you are putting a form together, you must remember that it's outline is based on framework components and helpers.
Start by wrapping your <form> tag into .atk-form div. This will unlock many additional features as described in this document. Additionally it will make input fields look nicer and consistent with the rest of your CSS theme.
Add a single .atk-form-fieldset div inside your form. This field-set will contain your field definitions.
<div class="atk-form">
<form>
<div class="atk-form-fieldset">
... fields ...
</div>
</form>
</div>