Apart from the noise from the outside world you also need to consider the 'sound of the room'. What I mean is does sound tend to bounce around your room as well? If you are recording in your bedroom you may find that you have all kinds of issues with the reverb on your vocals, which will give you less control as you mix down. If there are lots of hard surfaces (concrete walls - as opposed to plasterboard - windows etc), this may well be the case. Building a booth from room dividers might be a bit over the top - I have images of some poor singer being shut into a tiny dark booth, (with a roof!), built entirely out of office screens - however they might be useful for damping down some of the reflections in the room. You could place them in a V shape behind your singer. Make sure that your floors have rugs on if they aren't carpetted. Another trick is to hang bedding down the walls. Obviously this looks weird so I hope you're singer knows you! Basically you need to come up with some methods of cutting down reflections from hard surfaces in your room.
As for noise from the outside, following the above may help a bit. If it's very loud, like you're in the flight-path of your local airport there really isn't much you can do about it! However there are a few things you can do. I would recommend using a condensor mic, just because they sound so much better (it is actual singing you're doing, right?) Maker sure that you hit the low pass switch on the mic or your mixer - this will ensure that you cut out traffic rumble and the like. Try to get your singer to be fairly close to the mic (don't forget a pop-shield!) so that you don't have to turn the gain up too high. Possibly use a noise gate so that in silent parts any extraneous noise is cut out. If you're recording through a compressor be sparing! If your ratio is too high you will end up pulling up any noise as well.
Hope some of this helps.
KasioRoks
I'm going for a Mackie board (maybe 
