I want to have a plugin that will add a customized text on the article,

for example I add {additionalinfo} at the bottom of the article, there will be a sentence at the bottom

"[title of the current article] is a part of the [title of the Category of the current article]"

the [title of the current article] and the [title of the Category of the current article] are text link

is it possible?

To add any additional text, you would just append it to $row->text in Joomla1.0 or to $article->text in Joomla1.5.

Eg:

$article->text .= ' your custom text ';

Is this what you want?

may that's it but how do i put the customized text by just adding {additionaltext} on the article?

Try:

$custom_text = ' your custom text '; // your custom text
$article->text = str_replace('{additionaltext}', $custom_text, $article->text);

This will replace any instances of {additionaltext} with your $custom_text;

where should i put that code?

/public_html/?????

and also how do I add the title of the current article and category to my custom text?
for example

$custom_text = ' this *title of current article* is belongs to *title of category*';

Place the code in the plugin parameters, under the "onprepare" content event field.

The title is $article->title and the category would be $article->cat I believe.
You have to add those to your $custom_text variable.

eg:

$custom_text = 'this '.$article->title.' belongs to '.$article->cat;