What would happen if jupiter and saturn came together.?

>My science teach told me that jupiter only needs like 1 third more mass to be a sun

That is incorrect. In order to be a star, an object must be massive enough to sustain fusion in its core. Jupiter would need to be about 80 times more massive than it is in order for it to fuse hydrogen in its core.

That said, the energy of such a collision would be so enormous that the two merging planets may outshine the Sun in the sky. That much more light reaching the Earth could bake us all in our own atmosphere. Furthermore, even if it didn't, the collision would no doubt send chunks of material flying throughout the Solar System, and the Earth would undergo at least several million years of relatively intense impacting, which itself might be sufficient to destroy human civilization.

Jupiter would need a lot more mass than that to begin fusion. I think the lower limit of Brown Dwarfs is somewhere around 13 Jupiter masses. For sustainable fusion (red dwarfs) it's somewhere in the neighborhood of ( I think) 50 Jupiter masses.

The least massive main sequence star requires about 80 Jupiter masses. If Jupiter and Saturn collided at their mutual escape speed, the impact energy would be about 1e37 Joules, which is the amount of energy radiated by the sun in 828 years. So it's something that would Kill Us All.

It's a big misconception that a lot of people have, but Jupiter is not a failed star. Accordingly to my astronomy book that has a long chapter on this matter states that Jupiter would have to be 75 times more massive than it is in order to generate energy as the sun does.

That I don't know.

...no it would not make a new sun, just a planet of approx. equal size. But even if it did, it might be the last thing you see...

It would make a collisoal exploasion, so big that our neighboring galaxy would see it. All mankind would be whiped out. Also since the expolsion would be soo big it would cause the sun to age so rapidaly that it woulp implode, creating a black hole, which in turn suck everything and anything into it. Long story short. Bad things would happen, very bad things.