Implement an if in jekyll tag page

I have a page showing all the posts ordered by tag. The thing is that I only want some tags to be shown, not all of them. The thing would be after the "for tag in site.tags" implement an if like tag == x. I'm new to Jekyll and ruby and I don't know how to do it :/

<ul class="tags">

</ul>

<div>

</div>