Next, I notice you wrote the expression Point[{0,9/8}]. There are a couple of things to note here. First, Point is used for graphics. The cartesian point as data would be represented as just {0,9/8}. Also, just evaluating that expression doesn't do anything for you, and you can see that it's just replicated in the output. You might want to assign it to a variable at some point, but you don't need it here in this cell.

And that's another thing. It's generally more organized to separate separate conceptual steps into separate cells. So, maybe you put your function definitions into a cell and then put your derivative expression in a separate cell. It's not necessary or critical, but it helps keep things organized and avoids unexpected interpretations.

Finally, in case you haven't discovered it yet, you'll want to look at ParametricPlot when you get to the point of plotting this function.

Mathematica represents the constant pi as just Pi. When you're inputting Pi, just type it in as "Pi" (capitalized). You can represent multiplication as either * or just a space. And generally you would use the SetDelayed form to define your functions, at least this is probably best for Mathematica beginners. So, to define x you'd do something like

x[t_] := Cos[Pi t]

What was happening is that the special Greek form of pi that you were using was interpreted as just a regular symbol (well actually, it looks like there was no space, so the symbol was the "word" spelled with the letters pi and t.

I find it easier to not bother with special formatting for inputs. So your definition of y could be

y[t_] := t^3 + 2 t

I've added spaces to try to make things clear, but in this expression they are not required. Mathematica will figure out that a number followed by a symbol should be interpreted as multiplication.

What was happening was that you defined a function, x, in terms of t but the variable t didn't show up in the definition. So, x was effectively a constant, and taking its derivative in terms of t resulted in 0.