Category Conditinal Content

Hi I'm trying to include some category based conditional content in the theme. I'm working on the concept of: <?php if (is_category('this')) { ?> DO THIS <?php } elseif (is_category('that')) { ?> DO THAT <?php } else { } ?> How ever I'm not sure were to add the code. I'm not too ot with PHP so any happy would be great! cheers Andy