The first time Gojita appears in th Dragon ball Z, Movie 12. Gokou and Vegeta are fighting an enemy called Jeneba. The way that Gokou and Vegeta Fuse is the Fusion-Waza, also known as the Fusion Dance or " Fuuu-shion--Haaaa!! The first time the two tried to fuse Vegeta, instead of extending his finger, he put his fist out. Thsi fusion was done wrong and Gojita was formed but he was a Fat, slow, and weak. After 30 minutes the two unfused and tried the fusion again. This time the two fused correctly and killed Jeneba With ease.
Facts:

1) This fusion is not perminant and will wear off in 30 minutes.
2) The two people have to be the same height and almost the same power level.