Suppose you know that
$$
a\Gamma(a) = \Gamma(a+1) \tag{1}
$$
and that
$$
\int_0^1 x^{\alpha-1} (1-x)^{\beta-1}\,dx = \frac{\Gamma(\alpha)\Gamma(\beta)}{\Gamma(\alpha+\beta)}. \tag{2}
$$
From $(2)$, we get the probability density function of the Beta distribution with parameters $\alpha$ and $\beta$:
$$
\int_0^1 f(x)\,dx = \int_0^1 \frac{\Gamma(\alpha+\beta)}{\Gamma(\alpha)\Gamma(\beta)} x^{\alpha-1}(1-x)^{\beta-1}\,dx=1.
$$
Now we want the first and second moments $\mathbb{E}(X)$ and $\mathbb{E}(X^2)$ of a random variable $X$ with this density.
$$
\begin{align}
\mathbb{E}(X) & = \int_0^1 x f(x)\, dx = \frac{\Gamma(\alpha+\beta)}{\Gamma(\alpha)\Gamma(\beta)} \int_0^1 x \cdot x^{\alpha-1} (1-x)^{\beta-1} \, dx \\ \\
& = \frac{\Gamma(\alpha+\beta)}{\Gamma(\alpha)\Gamma(\beta)} \int_0^1 x^{(\alpha+1)-1} (1-x)^{\beta-1} \, dx. \tag{3}
\end{align}
$$
The integral identity in $(2)$ holds if $\alpha$ is any number at all; therefore it holds for $\alpha+1$:
$$
\int_0^1 x^{(\alpha+1)-1}
(1-x)^{\beta-1} \, dx = \frac{\Gamma(\alpha+1)\Gamma(\beta)}{\Gamma((\alpha+1)+\beta)}.
$$
It follows that the product in $(3)$ is
$$
\frac{\Gamma(\alpha+\beta)}{\Gamma(\alpha)\Gamma(\beta)} \cdot \frac{\Gamma(\alpha+1)\Gamma(\beta)}{\Gamma((\alpha+1)+\beta)}
$$
So $\Gamma(\beta)$ cancels, and $(1)$ can be applied to change $\displaystyle\frac{\Gamma(\alpha+1)}{\Gamma(\alpha)}$ to $\alpha$, and to change $\displaystyle\frac{\Gamma(\alpha+\beta)}{\Gamma((\alpha+1)+\beta)}$ to $\displaystyle\frac{1}{\alpha+\beta}$. Therefore the expression in $(3)$ simplifies to
$$
\frac{\alpha}{\alpha+\beta}.
$$
A similar technique differing only in details finds $\mathbb{E}(X^2)$.
Once you have $\mathbb{E}(X)$ and $\mathbb{E}(X^2)$, you can use $\operatorname{var}(X) = \mathbb{E}(X^2) - (\mathbb{E}(X))^2$. There's some algebraic simplifying to be done after that. Remember that the variance should be symmetric in $\alpha$ and $\beta$, so if what you get is not symmetric, there's a mistake.