The Google query strings bringing visitors to my blog are sometimes amazing, like the one in the title of this post. Let's start with a tangential remark: if you're serious about XSLT, you should invest into a good book (XSLT Programmer's Reference is one of the best technical books I've ever read). If, on the other hand, you're just looking for a quick fix, here's the whole story
XSLT does not have if-then-else or select-case constructs like most other languages. You can implement both functions with the xsl:choose element that can take as many xsl:when children as needed. These are evaluated sequentially, the first one that succeeds is executed. You can also use xsl:otherwise as the last child in the list to specify the default behavior.