What are the coordinates of a generator point? I'm browsing through Curve25519 code, the generator point of it is $G=9$. 

I would like to know how can I get $x$ and $y$ coordinates of this generator point. Is there any standard way of representing x and y as a single scalar value apart from $x$ over $z$ notation?
Elliptic curves live on a plane, each point having two coordinates. However, all points on a curve must fulfill the curve equation, which allows recomputing (most of) one coordinate from the other.

Let's see what it does for Curve25519. I am using notations from RFC 7748, as explained in sections 3 and 4.1. The two coordinates are then called $u$ and $v$, and the curve equation is:
$$ v^2 = u^3 + Au^2 + u $$
for constant $A = 486662$. Everything is computed modulo $p = 2^{255}-19$.

The generator point has coordinate $u_P = 9$. From the equation, we know that its other coordinate $v_P$ is then such that:
$$ v_P^2 = u_P^3 + Au_P^2 + u_P = 39420360 $$
Modulo $p$, that value has exactly two square roots, which can be computed with the Tonelli-Shanks algorithm; the values are:

v1 = 147816194475895447910205935684099868872 \
     64606134616475288964881837755586237401

v2 = 431144251710685529207648989359339670393 \
     70386198203806730763910166200978582548


Thus, when you say "$u = 9$", you narrow the choice down to exactly two points, $(9, v_1)$ and $(9, v_2)$. Conventionally, the first one is the actual generator.

As a further twist: it so happens that which of these two points you choose as generator does not matter, at least for the intended usage of that curve, i.e. to support Diffie-Hellman. Indeed, the two points $P_1 = (9,v_1)$ and $P_2 = (9,v_2)$ are such that $P_1 = -P_2$ (they have the same $u$ coordinate, and opposite $v$ coordinates). When doing Diffie-Hellman, the involves parties do the following:


Charles chooses a random secret $c$ and sends point $cP$.
Denise chooses a random secret $d$ and sends point $dP$.
Both compute $c(dP) = d(cP) = cdP$.
The shared secret is, conventionally, the $u$ coordinate of the latter point $cdP$.


Now, if Charles chooses $P_1$ but Denise works with $P_2$ has generator, Charles computes point $cdP_2$, while Denise makes $cdP_1$. But since $P_1 = -P_2$, we have $cdP_1 = -cdP_2$, and both points $cdP_1$ and $cdP_2$ have the same $u$ coordinate. Charles and Denise then end up with the same shared secret. Therefore, the exact choice between $P_1$ and $P_2$ does not matter.

This is the reason why the Montgomery ladder algorithm does not actually bother with the $v$ coordinate. All computations are done using $u$ only. Conceptually, when you do things with $u$, you are doing things with two points $(u,v)$ and $(u,-v)$ which are somehow mirror of each other. Since you keep only the $u$ coordinate at the end, the $v$ coordinate can remain implicit throughout.