I've never used MyBB, but I assume they have an area in the Admin Control Panel for adding custom BBcodes. One thing you need to know is that BBcodes need a HTML replacement. When you submit a post on a bulletin board, the forum converts all the BBcode into the HTML counterpart and then outputs that to your browser. So when you add a paragraph tag and a anchor/name tags, you also need to add the HTML replacement code.
The HTML for paragraphs is:
CODE
<p>{TEXT}</p>
and the BBcode usage is:
CODE
[p]{TEST}[/p]
Notice I put "Text" in brackets. This is called a token and the forum interprets that token as the stuff the user places within the BBcode. This might not even be the right token for MyBB forums, so consult the manual if it doesn't work. You can also add a style attributes to the paragraph tag using inline-CSS or add a class attribute to the HTML replacement and add the CSS attributes to the actual CSS document for the style. It's your choice.
Anchor tags are hyperlinks. Instead of jumping to the top of a page, they can be marked to jump midway on a page. This requires two BBcodes; an anchor tag and a name tag. The
anchor is the link to the area you're jumping to, and the
name tag is the area you want to jump to.
BBcode usage for anchor tag:
CODE
[anchor={SIMPLETEXT}]{TEXT}[/anchor]
HTML replacement:
CODE
<a href="#{SIMPLETEXT}">{TEXT}</a>
BBcode usage for name tag:
CODE
[name={SIMPLETEXT}]{TEXT}[/name]
HTML replacement:
CODE
<a name="{SIMPLETEXT}">{TEXT}</a>
SimpleText is another token and it's there to identify the difference between the two input areas for the end-user.