If you need to add more fields in your form then you can add new field in to the helloworld.xml
browse to the following file on to
administrator->components->com_helloworld->models->forms->helloworld.xml
Open this file you will find the number of fields are listed over there you can add your own field by copy any of the field and rename it as you want.
You can also refer this link : J2.5:Developing a MVC Component/Adding backend actions
For example you want to add description field on to your form then you just need to add this line between the fieldset
tag.
<field
name="description"
type="text"
label="COM_HELLOWORLD_HELLOWORLD_DESCRIPTION_LABEL"
description="COM_HELLOWORLD_HELLOWORLD_DESCRIPTION_DESC"
size="40"
class="inputbox"
default=""
/>
I would prefer you first read full tutorial of how to create MVC component for Joomla.Then you would be able to know how it works. Here the link :
J2.5:Developing a MVC Component/Developing a Basic Component
You may also use different form field type : Form field