SSJ Levels Achieved

Super Saiyan Goku
After Goku had hit Frieza with a huge Genki Dama, he and his friends celebrated. Then Goku realized that Frieza had survived, and shot Piccolo. Frieza then raised Krillin into the air and imploded him. This made Goku very angry, and he turned into a Super Saiyan!

Vegeta
Vegeta, being of royal saiyan blood, thought that he should have became the Super Saiyan of legend. He trained long and hard, but couldn't reach his goal. This made Vegeta very angry, and turned him into a Super Saiyan.

Gohan
Gohan was training with his father, Goku, in the Room of Spirit and Time. He realized that he had let down his friends during battles, and wouldn't be able to unless he turned Super Saiyan. He then turnesd into an awesome Super Saiyan.

Future Trunks
Future Trunks was the second Super Saiyan when he first appeared. He achieved this level in his own time. Gohan, the only remaining Z fighter, had knocked out Trunks, and went to take on the two Androids. Gohan was killed, and when Trunks found him, his anger made him a Super Saiyan.

Chibi Trunks
Chibi Trunks didn't have to do much to achieve Super Saiyan...when he had gained enough power, he went Super Saiyan to the suprise of Vegeta.

Goten
Much like Chibi Trunks, Goten could go Super Saiyan when he had enough power. This suprised Gohan.

Gotenks
Gotenks was in a sence "born" with the Super Saiyan ability. He was made when Goten and Trunks were SSJ, so he was born SSJ

Ultra Super Saiyan

Future Trunks
Future Trunks had achieved this level by training in the Room of Spirit and Time. He didn't learn the limitations of this level until he faught Perfect Cell.

Goku
Goku could achieve this level, but he refused to use it because he knew the limitations. He achieved this level while in the Room of Spirit and Time with Gohan.

Vegeta
Vegeta achieved this level, but not to the extent that Goku and Trunks did. Vegeta's muscle mass was increased, but not to the extent that his speed suffered too much unlike Goku and Trunks, who's speed greatly diminished.

Super Saiyan 2

Gohan
Gohan had first achieved this level in the Room of Spirit and Time, but he had it for only about 1 second. Then, while fighting Cell, Cell pissed him off by destroying Android 16, and releasing Cell Jr.'s to beat all his friends. This made Gohan angry enough to gain SSJ2 and hold it.

Goku
Goku reached this level while training in the after-life. He trained and battled with his ally and opponent Piku-han, and faught in the after life tournament with it resulting in a tie. So Goku gained enough to go Super Saiyan 2.

Vegeta
Vegeta, didn't actually achieve this level because he had to turn into Majin Vegeta to get this level. He wanted to beat Goku, and so he went turned into a Majin slave, and got enough power to go Super Saiyan 2.

Gogeta
Gogeta was "born" with the ability to go Super Saiyan 2. When Goku and Vegeta fused together, they were both SSJ2, so Gogeta was born SSJ2.

Vegetto
Vegetto, much like Gogeta, was "born" with the ability to go Super Saiyan 2. When Goku and Vegeta fused with the portoria earings, Vegetto was a regular saiyan/fusion. Then when Vegetto powered up to SSJ2.

Super Saiyan 3

Goku
Goku, just like SSJ2, had reached this level in the afterlife while training with Piku-han. He used it on Buu, but couldn't reach full power, and this level made him go back to the after-life quicker.

Gotenks
Gotenks gained SSJ3 while training with Piccolo in the Room of Spirit and Time

Super Saiyan 4

Goku
While fighting, Bebi Vegeta, Chibi Goku was getting a whoopin. He was knocked almost unconcious, and then looked at the full Earth. At that point, Goku changed into the Golden Oozaro. He was a beast, and couldn't be controled. He then powered up in the Golden Oozaro state, and turned into SSJ4 Goku.

Vegeta
Bulma, his wife developed a Brute Ray, a machine that would give Vegeta the ability to go Golden Oozaro. So in a sence, Vegeta cheated.

Gogeta
Gogeta achieved this state by Goku and Vegeta, both in thier SSJ4 state fused together to beat the evil Li shenron. Unfortunatly, because he was so powerful, the fusion didn't last the usual half hour, it went for about 15 to 20 minutes.