To make a picture look natural, you have to have to be able to use expressions properly. In Goten's case, it's pretty important to learn how to make him look innocent, and as Goten's natural seems to be happy, I'll go over that first.
The first above image is Goten looking normal, the second, Goten surprised/happy, and the third cheeky/happy. Combinations of more than one emotion are common, but you should be careful to keep the emotions in character. For example, the below right image is angry/jealous, and looks more than a little out of place.
 

(The second and third images are angry, and angry/determined) When drawing angry faces, the eyebrows narrow and the eyes are sometimes 'boxed in'. The lower eyelids often rise a little as they do in a lot of the more expressive emotions (this also happens when happy, if fact the infamous ^_^ expression is an exaggeration of it). One of the hardest parts to draw when drawing angry expressions is drawing clenched teeth. To stop the mouth from looking like a grin try making the bottom lip curve upwards a bit, and round the top lip down at the sides.
 

A character can be made to look confused by using one 'frowning' eyebrow and one 'sad' eyebrow, as well as an 'O' shaped mouth. Goten pulls this expression a lot...
By using eyebrows bent upwards, you can make the character look sad or apologetic. Notice the difference moving the mouth upwards makes in the third above fanart.

Seemingly little differences can make a big difference to how the emotion is read. The first two images use the same eyes and eyebrows, but the shape of the mouth makes the two completely different. (The first is crying, the second is a sneeze or a yawn). You don't have to learn each expression individually, as long as you know how to do the basic types of eyes and mouths, you'll find you can figure out most of the emotions as you go.