There are several scoring systems used to evaluate, judge and compare wines. The two most common are the "UC Davis 20-point scale system" (http://www.finias.com/wine/ucd_scoring.htm) many derivatives of which exist, and the 100-point "Parker System" (http://www.erobertparker.com/info/legend.asp). 

That said, there as as many opinions and articles on scoring wines as there are wine tasters, and you may end up devising your own way of evaluating wines, but for the sake of being able to compare your findings with the people around you, it is recommended that you familiarise yourself with at least one of these two systems.

The derivative of the 20-point UC Davis System which is commonly used in South Africa is as follows:

The scale allocates 3/20 points for colour and appearance, 7/20 points for the nose of the wine and 10/20 points for the palate and finish.

19-20: Superlative

17-18: Excellent

15-16: Good to Very Good:

13-14: Average

12:      Poor

0-11:   Faulty

Here is what Robert Parker has to say about his 100-point system:

 "It is my belief that the various twenty (20) point rating systems do not provide enough flexibility and often result in compressed and inflated wine ratings."  

His scale follows below: 

The scoring system gives every wine a base of 50 points. The wine's general color and appearance merit up to 5 points. Since most wines today are well made, thanks to modern technology and the increased use of professional oenologists, they tend to receive at least 4, often 5 points. The aroma and bouquet merit up to 15 points, depending on the intensity level and dimension of the aroma and bouquet as well as the cleanliness of the wine. The flavor and finish merit up to 20 points, and again, intensity of flavor, balance, cleanliness, and depth and length on the palate are all important considerations when giving out points. Finally, the overall quality level or potential for further evolution and improvement—aging—merits up to 10 points.

96-100: An extraordinary wine of profound and complex character displaying all the attributes expected of a classic wine of its variety. Wines of this caliber are worth a special effort to find, purchase, and consume.

90-95: An outstanding wine of exceptional complexity and character. In short, these are terrific wines.

80-89:   A barely above average to very good wine displaying various degrees of finesse and flavor as well as character with no noticeable flaws.

70-79: An average wine with little distinction except that it is a soundly made. In essence, a straightforward, innocuous wine.

60-69:   A below average wine containing noticeable deficiencies, such as excessive acidity and/or tannin, an absence of flavor, or possibly dirty aromas or flavors.

50-59: A wine deemed to be unacceptable.