(You can use (single or double) quotes or double square brackets for params with spaces)
Notes
This is intended to help you get started with customising your TW. To make the macro work you have to give this tiddler a tag of systemConfig then save and reload. To learn more about customising Tiddlywiki? Look at other people's plugins or... click View, Source in your browser and start reading. :)
Code
config.macros.helloWorld = {};
config.macros.helloWorld.handler = function (place,macroName,params,wikifier,paramString,tiddler) {
var who = params.length > 0 ? params[0] : "world";
wikify("Hello //" + who + "// from the '" + macroName + "' macro in tiddler [[" + tiddler.title + "]].", place);
}