If you read the post on tasting red wines then you will see some similar information here, but the post has been revised for white wines.
To begin, look at the color. Consider how the color compares with other wines of this varietal that you have had. White wines can be nearly colorless all the way to a deep amber hue. Does the color of the wine seem similar to other wines of that varietal that you have had? If a wine is leaning more toward that amber color then it is possible that the wine is oxidized. Oxidized wine has a sherry character and is considered a fault in wines other than sherries. Knowing this before you taste a wine is helpful information to have. It tells you that the wine could either be an older vintage, or it was stored incorrectly with excessive exposure to heat, oxygen, or both.
Consider the temperature of the wine. Is this wine being served right out of the fridge, or is it at room temperature? It is recommended that white wines are served in the 40-60°F range (40-50°F for light body whites, 50-60°F for fuller bodied whites) so it is good to know if the wine is cooler, warmer, or at the recommended serving temperature. A wine that is too cold will not be easy to taste. The fruitiness will be muted. If a wine is too warm then the fruitiness might seem excessive.
Now take your first smell of the wine. I like to do this before I swirl the glass. Think about what you smell. Now go ahead and give it a swirl and smell it again. Do you smell fruit? what kind of fruit? Is it cooked fruit, or fresh fruit? If it is fresh fruit is it citrus fruit, tree fruit, or tropical fruit? If it is citrus fruit, is it grapefruit, lemon, lime? Or if it is tree fruit, is it apple, pear, peach, or nectarine? Is there an oaky character? Is it a raw wood smell, or a toasted smell? Do you smell vanilla, caramel, clove, or chocolate? These are all aromas that are usually derived from an oak influence so they are indicators that the wine spent some time in barrel, or in contact with oak.
Now taste the wine. What is the first thing that stands out? Do you taste the same fruit that you smelled? Is the wine astringent, or bitter? What is the mouthfeel like? Is the wine balanced between the tartness and the sweetness? That is not to say that a wine is sweet, but the fruit character, and oak can contribute a perception of sweetness that can help balance out the sour and bitterness that wines can have.
Once you swallow the wine what happens? Did you taste flavors throughout the sip? Did it feel like you tasted it when you first put it in your mouth, maybe there wasn’t much in the middle, but you tasted it again when you swallowed it? What is the finish like? The finish is what you taste, and the sensations you have in your mouth after you swallow the wine. It is the after effect. Does it linger? Can you taste the wine for a while? Or does it go away as soon as you swallow the wine? Is there a burning sensation in your throat? That is generally an alcohol burn. It is the sensation that happens when you swallow a higher alcohol wine, or when you drink spirits. Is there a bitterness in the finish? This is similar to the taste that an aspirin leaves in your mouth if you don’t drink enough water with it.
I like to take a few sips and really consider all of this with each wine I am tasting. You may want to start keeping notes about the wines you taste. By taking time to think about what you smell and try to identify the aromas will help you build your wine vocabulary.