Before you get Goku, go to the northern mountains. Find the barrier that Goku is supposed to break. There should be a dinosaur behind it. Hit it through the fence. You will gain a lot more if you are under level 30.
If you are finding it difficult to level up your characters, the best place to train them would be the "ninja castle" where you fought General Tao while searching for the Dragonballs. Keep fighting the ninjas and they will give you a reasonable amount of experience points.
To level up easily, go to the Snow Mountains. In the upper left corner will be a black mark. When you travel to it, it appears that you cannot get in, but the wall looks slightly cracked. Punch it and there will be a dinosaur. Kill it, get the capsule, and use it. Leave, kill the monsters outside, then go back in and repeat as many times as desired.