The broken spoke will be easiest to get at by removing the wheel, you then need to take off the tyre and tube. Hopefully the broken one is inserted from the inside of the hub as these are the easiest to replace and the last to ones to go in.
Half of the spokes (the 1st ones) are inserted through hub form the outside and then the other half from inside of the hub. Obviously you can get the correct spoke length once you have both bits to measure. Source from ebay or the bike shop where you got your bike. When fitting spokes don't be afraid of flexing spokes to get them in as you will usually have to cross them over and under (depending on cross pattern) any slight bend in them will straighten once they are tensioned.
If you do replace it yourself and tighten spokes you can gauge the tension by pinging the spokes so that they give a nice pling similar to the others.