set margin for forms

First up, you're missing the closing form tag in your HTML. It won't really affect how your CSS styles make the page look, but it should really be in there.

Second, all of those input elements are inline by default. That is, they flow like normal text.

Are you wanting them to each be on their own line, with margins to separate the fields? If so, then each of those inputs needs to have the CSS property and value of display: block.

Ideally, you wouldn't style using an ID for every single element, as this will be difficult to maintain in future. Try giving the parent form tag a class and then styling all text inputs that are descendants:

<form action="index.html" method="post" class="contact">
...
</form>

.contact input[type="text"] {
  display: block;
  margin-top: 50px;
}

Alternatively, you can group inputs that are styled the same using classes in your HTML:

<input type="text" id="first_name" name="user_first_name" class="font-size-medium">
<input type="text" id="last_name" name="user_last_name" class="font-size-medium">
<input type="text" id="email" name="user_email" class="font-size-large">
<input type="text" id="street_name" name="street_name" class="font-size-medium">
<input type="text" id="city" name="city" class="font-size-large">
<input type="text" id="province" name="province" class="font-size-medium">
<input type="text" id="postal_code" name="postal_code" class="font-size-medium">

.font-size-medium {
  font-size: 20px;
}
.font-size-large {
  font-size: 25px;
}

If I've misunderstood how you would like to format your inputs, please either try and explain exactly how you would like it to appear, or perhaps try create and share a basic diagram somewhere (Google Drawings on Google Drive/Docs or something?).