Understanding The Fundamental Theorem of Calculus, Part 1 First Part of the Fundamental Theorem of Calculus says that "the derivative of a definite integral with respect to its upper limit is the integrand evaluated at the upper limit."
So it means that the antiderivative of integrand evaluated at the upper limit is it's integral? Can you please explain for me how it works?
So it says that for
$$g(x) = \int_a^x f(t) dt$$
$$\frac{d}{dx}g(x) = f(x)$$
So does it mean that we can find an integral from a to x, by finding antiderivative of f(x)? If so where does a go?

You asked whether it means:

...we can find an integral from $a$ to $x$ by finding antiderivative of $f(x)$?

No, this is not what it means. What is means is exactly the opposite:

... we can find an antideritive of $f(x)$ by finding an integral from $a$ to $x$.

You can tell that's what the theorem says if you pay attention not only to the conclusion of the theorem, but also its hypothesis. To add to the excellent comment of @geetha290krm, it's a bad idea to state a theorem in math avoiding its hypothesis.
The hypothesis of this theorem is:

Let $f : [a,b] \to \mathbb R$ be a continuous function.

Right away you can see: you already have the function $f$ in your hand. Your goal is to find an antiderivative of it. And the way you find that antiderivative is given in the conclusion of the theorem:

For $g(x) = \int_a^x f(t) \, dt$, we have $\frac{d}{dx} g(x) = f(x)$.

