Passing Variables across functions

If you want something from a function it should return a value. Local variables are called local because they are only visible in the context they were defined in. I'll try to show you where you erred:

// 1. function declaration don't go inside main
void up(float& speed); // This one's defined twice! Why?
void down(float& speed); // It's been declared allright but defined
void final();


int main(int argc, char *argv[])
{
    float speed;
    cin << speed;
    up(speed); // function speed is void so it doesn't return anything
    //down(speed); You did supply a declaration but not a definition
    //right(speed); You didn't supply a declaration and neither a definition
    final(); // This function doesn't have any parameters so it can't know the values that were not returned by the functions
}
void up(float& speed) // should return float
{
     float one;
     if(speed = 1)
     {
        one = 10;
     }
     else if(speed = 2)
     {
        one = 20;
     }
     // return one;
}

//Duplicate function definition with the same name and signature
void up(float& speed)
{
     float two;
     if(speed = 1)
     {
        two = 10;
     }
     else if(speed = 2)
     {
        two = 20;
     }
}

void final() // function does not have parameters and one and two are not global variables, so one and two in the functions body are not defined
{
     cout << one + two;
     //---- This is the bit I dont get. I want the variables one and two from the previous functions but I have already passed 'speed' through and am not sure how to do this. Please help!
}

What you wrote really shows a total lack of understanding of what you are doing. Did you event try to compile that. If you had the compiler should have given you enough hints on what to improve.

void up(float& speed, float& one, float& two);
void down(float& speed, float& one, float& two);
void final(float& one, float& two);
int main(int argc, char *argv[])
{
      float speed;
      float one;
      float two;

      cin << speed;

      up(speed,one,two);
      down(speed,one,two);
      right(speed,one,two);
      final(one,two);
}

void up(float& speed, float& one, float& two)
{
      if(speed = 1)
      {
            one = 10;
      }
      else if(speed = 2)
      {
            one = 20;
      }
}

void up(float& speed, float& one, float& two)
{
      if(speed = 1)
      {
            two = 10;
      }
      else if(speed = 2)
      {
            two = 20;
      }
}

void final(float& one, float& two)
{
      cout << one + two;
}