Using a hydrometer

There are many guides as to how to use a hydrometer; I won't repeat the content, but links are given below. The essentials are:

• Measure at the meniscus before you pitch yeast, and record the value; this is your original gravity (OG). It'll be a reading like "1056" or "1095".
• Measure at any point afterwards; the alcohol percentage in what you have is (OG - current reading) / 7.45.
• The reading at the end of fermentation is FG, for final gravity.

Pretty direct guides:

• http://www.grapestompers.com/articles/hydrometer_use.htm
  
• http://www.countrybrewer.com.au/webcontent35.htm

The advanced techincal guide has to be http://winemaking.jackkeller.net/hydrom.asp.