Use 我对你想说 or 我想说对你 to say: I want to talk/speak to you?
It should be 我想对你说

我想说 = I want to say

我想(对你)说 = I want to say (to you)


对 have to be placed before the object, and  [对 + object] have to be followed by a verb
想(want to) is the main verb, it is not moved to the end of the sentence
O Connor wrote:

Is it because of the word 对 that you need to move the second verb 说 to the end of the sentence?

Not necessarily the end of the sentence, but [对 + object] need to be followed by a verb

我说 = I say

我(对你)说 = I say (to you)

我现在(对你)(说)明天的计划 = I now say (to you) tomorrow's plan