In general, let $L$ be a line bundle on a complex torus $X=V/ \Lambda$ of dimension $g$ and let $H$ the hermitian form corresponding to its first Chern class. The imaginary part $E:= \textrm{Im}(H)$ is an alternating form which is integer-valued on the lattice $\Lambda$. 

By elementary linear algebra there is a basis of $\Lambda$ with respect to which $E$ is given by the matrix $$\left(\begin{matrix}0  &  \Delta \cr - \Delta & 0 \end{matrix}\right),$$ 
where $D=\textrm{diag}(d_1, \ldots, d_g)$ and the $d_i$  are strictly positive integers satisfying $d_i|d_i+1$ for all $i=1, \ldots ,g-1$.

If $L$ is positive-definite then one shows that
$$h^0(X, L)=\textrm{Pf}(E)=\det(D).$$
The proof consists in explicitly writing a basis for $H^0(X, L)$ by using canonical theta functions, as in Sebastian answer.

Saying that $\Theta$ is a principal polarization is equivalent to say that $D$ is the identity matrix, so $h^0(X, \Theta)=1$.

See [Birkenhake-Lange, Complex Abelian Varieties, Chapter 3] for further details.