Here is a ready-to-use contact form, which you can add to your Netlify site with Bootstrap. It will just work:
<form name="contact" method="POST" data-netlify="true" data-netlify-recaptcha="true">
<div class="form-group">
<label for="name">Name:</label>
<input type="text" class="form-control" id="name" name="name" required>
</div>
<div class="form-group">
<label for="email">Email:</label>
<input type="email" class="form-control" id="email" name="email" required>
</div>
<div class="form-group">
<label for="subject">Subject:</label>
<input type="text" class="form-control" id="subject" name="subject" required>
</div>
<div class="form-group">
<label for="message">Message:</label>
<textarea class="form-control" rows="10" id="message" name="message" required></textarea>
</div>
<div class="form-group">
<div data-netlify-recaptcha="true"></div>
</div>
<button type="submit" class="btn btn-primary">Submit</button>
</form>