You could use CSS in relation to this, but it's not specifically a CSS problem. Something as simple as this should work:
<label for="last_name">Last Name:</label> <input type="text" id="last_name" name="last_name" />
<label for="first_name">First Name:</label> <input type="text" id="first_name" name="first_name" />
<br />
<label for="phone">Phone Number:</label> <input type="text" id="phone" name="phone" />
As I said, CSS could be related, for example if you have inputs or labels defined as display:block instead of display:inline, line-breaks will be added, but it shouldn't happen by default.