Unirational and ampleness Let U be a unirational variety over a field of characteristic 0. I have read that its canonical divisor cannot be ample, but I don't know why.
Any references will be most helpful.
 A: Smooth varieties 
Let $X$ be a smooth uniruled variety. Then there is a free rational curve $f:\mathbb{P}^1\rightarrow X$, that is $H^1(\mathbb{P}^1,f^*T_X\otimes\mathcal{O}_{\mathbb{P}^1}(-1)) = 0$. Now $f^{*}T_X$ is a rank $n = dim(X)$ vector bundle on $\mathbb{P}^1$. Therefore we can write
$$f^*T_X = \mathcal{O}_{\mathbb{P}^1}(a_1)\oplus...\oplus\mathcal{O}_{\mathbb{P}^1}(a_n).$$
Now, $f^{*}T_X$ contains $T_{\mathbb{P}^1}\cong\mathcal{O}_{\mathbb{P}^1}(2)$. We have $K_X\cdot f_*\mathbb{P}^1 = -\sum_{i=1}^{n}a_i\leq -2$.
Therefore, $X$ uniruled implies that there is a free rational curve on $X$ which implies that $K_X$ is not nef. So, if $K_X$ is ample (in particular $K_X$ is nef) then $X$ is not uniruled. 
Now, $X$ unirational $\Rightarrow$ $X$ rationally connected $\Rightarrow$ $X$ uniruled $\Rightarrow$ $K_X$ is not nef (in particular $K_X$ is not ample).
In other terms. If $X$ is uniruled then $X$ has nagative Kodaira dimension. If the canonical bundle of $X$ is ample (it is enough big) then $X$ has Kodaira dimension $dim(X)$.  
Singular varieties
There are singular uniruled varieties with ample canonical bundle. In particular there are many examples of rational singular surfaces with ample canonical bundle. The following example is due to Kollar. You can find the details and many other interesting constructions here http://arxiv.org/abs/1007.1936
Let us consider the surface
$$Y = Y(a_1, a_2, a_3, a_4) := (x_1^{a_1}x_2 + x_2^{a_2}x_3 + x_3^{a_3}x_4 + x_4^{a_4}x_1 = 0)$$
in the weighted projective space $\mathbb{P}(w_1, w_2, w_3, w_4)$, such that
$$a_1w_1+w_2 = a_2w_2 + w_3 = a_3w_3 + w_4 = a_4w_4+w_1 = d$$
and
$$\begin{array}{ll} w_1 = \frac{1}{w^*}(a_2 a_3 a_4-a_3 a_4+a_4-1),
& w_2 = \frac{1}{w^*}(a_1 a_3 a_4-a_1 a_4+a_1-1),\\
  w_3 =
\frac{1}{w^*}(a_1 a_2 a_4-a_1 a_2 + a_2-1), &  w_4 =
\frac{1}{w^*}(a_1 a_2 a_3- a_2 a_3 + a_3-1),
\end{array}$$
$$d = \frac{1}{w^*}(a_1 a_2 a_3 a_4-1)$$ where $w^* = \gcd(w_1, w_2,
w_3, w_4).$ If $w^* = 1$, then $Y$
is a rational surface with $4$ cyclic singularities and $H_2(Y, \mathbb{Q}) \cong
\mathbb{Q}^3$. The rational curves
$$C_1 := (x_1 = x_3 = 0),\quad C_2 := (x_2 = x_4 = 0)$$ are extremal rays
with respect the $K_Y + (1- \epsilon)(C_1 + C_2)$ minimal model program for
$0 < \epsilon \ll 1$. Therefore $C_1$ and $C_2$ are contractible
to quotient singularities and we get a rational surface of Picard
number 1,
$$\pi : Y=Y(a_1,a_2,a_3,a_4) \rightarrow X = X(a_1, a_2, a_3, a_4).$$
If $a_1, a_2, a_3, a_4 \geq 4$, then $K_{X}$ is ample.
For examples of big surfaces with big cotangent bundle you could take a look at this http://www.latp.univ-mrs.fr/~eroussea/big-surfaces.pdf
For a classification of complex compact Gorenstein surfaces with negative canonical bundle you can read this http://link.springer.com/article/10.1007%2FBF01421952#page-2. A classical example of a rational surface with negative canonical bundle is the quadric cone in $\mathbb{P}^3$. The canonical divisor $K_S$ is $-2H_{|S}$, where $H$ is a plane in $\mathbb{P}^3$ through the vertex.
