To replace a spoke you're going to need a spoke wrench of the correct size, a nipple driver or flathead screwdriver, a ruler to measure the correct spoke length, as well as some lube to put inside the spoke nipple. When you need to replace a spoke, using a ruler like this will make it easier to measure the length of the spoke that you require. It has a little hole at this end to put the spoke end into.
Once you've measured out your new spoke, you're going to slide it into the hub. Once you have the spoke in place, you're going to have to examine the old spoke pattern to figure out how you're going to install your spoke. Once you have it in the right direction, now you need to get the spoke nipple. The spoke nipple goes through a whole on the back of the rim where it came out of. Now that you have the spoke nipple in place, you can take either a spoke driver or flathead screwdriver and start to screw it in.
Before you tighten it down all the way you want to press down at the hub to pre-bend the spoke.
Before you drive the spoke nipple too far, add a couple drops of a lubricant to it. Now you can take the spoke wrench- you must select the right size- and continue tightening it by turning it clockwise. Now after you've finished tightening your spoke just give it up pluck and pluck some of the other similar spokes beside it, and then you can judge by the sound.