Dynamic forms

Hey all, sorry if in wrong thread.

Looking to do a dynamic form, i know java and ajax are used but they really confused the buggery out of me!!

Hate it ...

So i was wondering what's the simplest way to be done.

<form id="form3" name="form3" method="post" action="">
						<label for="name" class="text1">Name</label>
						<br />
						  <input name="name" type="text" class="input1" id="name" size="30" />
						</p>
						<p>
						  <label for="email" class="text1">Email</label>
						<br />
						  <input name="email" type="text" class="input1" id="email" size="30" />
						  <br />
						  <label for="email2" class="text1">Verify Email</label>
					  <br />
							<input name="email2" type="text" class="input1" id="email2" size="30" />
							<label for="memberyesno" class="text1"></label>
							<br />
							<label for="allreadymember" class="text1">Member Name</label>
							(Needed for faster Response)<br />
							<input name="allreadymember" type="text" class="input1" id="allreadymember" size="30" />
							<br />
							<label for="reason" class="text1">Reason<br />
						  </label>
							<select name="reason" class="input1" id="reason">
							  <option selected="selected">Choose One</option>
							  <option value="How Do I?">How Do I?</option>
							  <option value="Moderator">Become a Moderator</option>
							  <option>Forgotten Password</option>
							  <option value="Abusive Member">Abuse From another Member</option>
							</select>
							<br />
							<label for="comment" class="text1">Comment<br />
							</label>
							<textarea name="comment" cols="50" rows="5" class="input1" id="comment"></textarea>
						</p>
					  </form>

Thats my form code.

I want to make it so when the user is a registered member they tick a tickbox and a textfield appears so they can put there member names in it.

Ive researched code and it just confuses me!

Much Help is appreciated!

Ben