Se means if and quando means when. There is no difference between English and Portuguese concerning the meaning of these two conjunctions. However sometimes both conjunctions are possible from a grammar point of view, although the meaning is not the same.
When he comes, I will ask him.
If he comes, I will ask him.
In the first sentence he will come, however it is perhaps unclear when. In the second sentence it is not even sure that he comes. Choose in the following sentence the more plausible option.