If you only want to search a particular post type include a hidden field with the name post_type and the value set as the name of the post type you want to search:
<form role="search" method="get" action="https://wordpress.stackexchange.com/">
<input type="text" name="s" placeholder="Search">
<input type="hidden" name="post_type" value="post_type_name">
<input type="submit" value="Go">
</form>
Just replace post_type_name with the actual name of your post type.
Now when you search the URL will look like this:
http://example.com/?s=search+term&post_type=post_type_name
And only return results from that post type.