To start, you will need to set the form parameters using <form> and closing it off with </form>.

Next comes the part where you leave a space for others to fill out. For example, I am making a form for affiliate applications:

<form method="post" enctype="text/plain" action='mailto:email@whatever.com'>
<input type="hidden" name="To" value="Affiliation">
Your Name: <input type="text" name="Name"><br>
Site Name: <input type="text" name="Site"><br>
Site Address: <input type="text" name="Address"><br>
Button URL: <input type="text" name="Button"><br>
Comments: <input type="text" name="Comments">
</form>

This then looks like:

Your Name:
Site Name:
Site Address:
Button URL:
Comments:

The "name" function will show up when the form is emailed to you. So if the person wrote that her name was Ella, then this will appear on your email as Name: Ella.

But of course you don't want the applicant having to squish his/her comments into that tiny box, so you can change that line to this:

Comments: <br><textarea name="Comments" rows="10" cols="40"></textarea>

This will than make it look like:

Comments:

However, if you just want to increase the length of the type box and not the height, then you can use the following:

Your Name: <input type="text" name="Name" size="35">

And this will make it look like:

Your Name:

The default size is 20. The larger the number the longer the length.

Additions
There are also other stuff you can add to your form to make it more detailed when you're looking over whatever it is that's been sent to you. First, let's start with single-choice selections.

<form>
<input type="radio" name="Type" value="Affiliate"> Affiliate<br>
<input type="radio" name="Type" value="Partner"> Partner
</form>

We would then see something like this:

Or if you preferred to have more than one selection, you can use the check box:

<form>
<input type="checkbox" name="Site Theme" value="Graphics"> Graphics<br>
<input type="checkbox" name="Site Theme" value="Neopets"> Neopets<br>
<input type="checkbox" name="Site Theme" value="Tutorials"> Tutorials<br>
<input type="checkbox" name="Site Theme" value="Resources"> Resources<br>
</form>

This will show up as the type of website the applicant owns:

And finally, to round it all off, a submit button to send that form to your email:

<input type="submit">

If you want to be extra fancy, you can also choose to have a reset button right next to that as well:

<input type="submit" value="submit">
<input type="reset" value="reset">

Final Product

The code for the above form is as follows:

<form method="post" enctype="text/plain" action='mailto:email@whatever.com'>
<input type="hidden" name="To" value="Affiliation">
Your Name: <input type="text" name="Name" size="35"><br>
Site Name: <input type="text" name="Site" size="35"><br>
Site Address: <input type="text" name="Address" size="35"><br>
Button URL: <input type="text" name="Button" size="35"><p>
What are you applying for?<br>
<input type="radio" name="Type" value="Affiliate"> Affiliate<br>
<input type="radio" name="Type" value="Partner"> Partner<p>
What kind of site do you own?<br>
<input type="checkbox" name="Site Theme" value="Graphics"> Graphics<br>
<input type="checkbox" name="Site Theme" value="Neopets"> Neopets<br>
<input type="checkbox" name="Site Theme" value="Tutorials"> Tutorials<br>
<input type="checkbox" name="Site Theme" value="Resources"> Resources<p>
Comments: <br><textarea name="Comments" rows="10" cols="50"></textarea><br>
<input type="submit" value="submit">
<input type="reset" value="reset">
</form>

Remember to change the email address highlighted in bold to your email address. Please note that email forms are just like email links, when you send your info, the email address is automatically sent by the email connected to your server. I strongly recommend you do not use this as a lot of kids would not be aware that they are using their parent's email when they press send.