How can I convert this negative fraction to a positive one?

For any real numbers $a$ and $b\neq 0$, we have $-\frac{a}{b}=\frac{-a}{b}=\frac{a}{-b}$. Also, $-\frac{-a}{b}=-\frac{a}{-b}=\frac{-a}{-b}=\frac{a}{b}$.

Realize that negative signs represent multiplying the number by $-1$. Any fraction can be multiplied by $\frac aa$ without changing the value, since it equals one. So, multiply your fraction by $\frac {-1}{-1}$ to clear negative signs (or to move them between the top and bottom).

I think that $-\frac{x}{-2}$ is equal to $\frac{-x}{-2}$, is this true?

Yes, indeed: $$-\frac{x}{-2} = -1 \times \frac{x}{-2} = \frac {-1}{1}\times \frac x{-2} = \frac{-x}{-2}$$

But you can simplify further: $$\frac{-x}{-2} = \frac{-1\times x}{-1\times 2} = \frac x2$$ by cancelling $-1$ of in the numerator and denominator.