If you want to know the alcohol percentage of your beer, take an SG reading using your hydrometer before fermenting (Original Gravity (OG)) and then another after the beer is ready to be bottled (Final Gravity (FG)).
To calculate the percentage of beer, simply put your OG and FG into the following equation:
ABV = (OG-FG) x 131
i.e.
ABV = (1.056-1.014) x 131
ABV = 0.042 x 131= 5.502%