There are a number of informative web pages available (I included two, but not necessarily the best).
To do the job right, you need the proper tools. A good set of files is one of the first things you will need. -
http://www.guitarpartsresource.com/tools.htm
You can also find sets at Amazon and other web sources.
The first thing I would do is watch some videos so you can determine if you need to replace the nut.
If you decide to replace the nut, then you have to determine material (the great debate begins). I will give you my spin or opinion on material. If the slots and and height are good, material makes little difference as it pertains to tone. This is always debated, but once you fret the strings, the nut is really taken out of the picture. However, the nut also plays a role in wear and that is where plastic tends to lose out when compared to other materials.
Many people prefer bone for what they say is tone and appearance. Although bone will wear over time (tuning and restringing wear), if treated correctly, it will last for many years. Adding a dry lubricant such as a graphite will help wear and also helps maintain tuning. I use a graphite made for door locks, but there are many different types on the market.
Another material that people like to use is tusq. I tried this material on an acoustic saddle and didn't care for it and have since avoided its use. However, many people use it and swear by it.
http://www.fretnotguitarrepair.com/nuts.htm
http://www.ehow.com/video_2375091_adjusting-guitar-nut-height.html
In the end, you might decide the investment in time and tools isn't worth the hassle and then it's time to visit Woodsy's and have them do it.