Hello,
Thanks for the question. Since you did not give us any particulars on how you're creating a website, I'm going to assume you're using Joomla (since you posted the question in the Joomla section). Joomla does come with Bootstrap loaded by default. If you want to use it on a webpage on your site, then you simply add the code to that page and modify it to meet your content needs. You can find a great summary of Bootstrap code that you can use here: Bootstrap home page.
Here's an example of using Bootstrap code that you can insert into the web page (just like your examples above):
<div class="alert alert-danger" >
<p> This is going to be an alert in red </p>
</div>
The code then looks like this on the page
This is going to be an alert in red
The above example shows boostrap code with content added and demonstrates how it would appear (Bootstrap works in our Support Center as well). You can also find more detailed information on our Support Center about this issue. Try going to Add Bootstrap to Template in Joomla.
I hope that helps to answer your question. If you require further assistance, please provide more information on your problem and we would be glad to help.
Regards,
Arnel C.
This is going to be an alert in red