Their usage is almost the same. The only real difference is the fact that "tell" includes an indirect object. I said I wanted pizza. I told you I wanted pizza. Here, you can see how tell includes an indirect object in the sentence structure. Basically, while you can say something, you have to tell something to someone else.

I told you to pick up the trash ( tell):أي أمر يلزمه فعل I say or I said : I said I will be there at 8. (عندما تتكلم عن نفسك)