Well, you just got your camera and lens, so I recommend you get used to them first and foremost...before you even think about considering your next lens purchase.
But for portraits, have a look at the ultra cheap and plasticky, EF 50mm f/1.8 II if you're on a budget. This lens may be dirt cheap ((US$85), but it has very sharp image quality. And, it's great to use in lower light (much better than your kit lens).
Or, the EF 85mm f/1.8 USM. This lens is an overall better lens (a lot better build, metal mount and USM), so accordingly, costs more (around US$330) . I bought one of these when I bought my 450D (XSi) and am VERY pleased with it's performance (bokeh is also very nice).
For Macro (and it also makes an excellent portrait lens), you can't go past the very sharp, EF100mm f/2.8 USM Macro. This lens retails for under US$450 and is definitely on my future lens buying list.
For landscape, you should just try using the wide end of your kit lens for now...and later on, when you can afford it, buy a wide angle lens like the EF-S 10-22mm f/3.5-4.5 USM.
A word of advice. Lens choice is a very important factor if you want VERY good image quality. Do not be tempted to save a few dollars by buying an ultra cheap (telephoto) lens because you usually get what you pay for...
Another thing to consider is filters. Buying a UV filter to protect the end of your lens makes sense. Always leave it attached. And, maybe later you could try a CP (circular polarizer).
check out the reviews of all these lenses in the links below...
good luck