Gaussian noise definitions
The text:


  The measurements are influenced by gaussian noise with center in 0V and standard deviation equal to 3 Volts. 


simply means that the Gaussian noise has $\mu = 0$ (the mean is zero) and $\sigma = 3$ (the standard deviation is 3 volts).


  Determine the model for the system, as well as initial values to implement a Kalman Filter


Without more information, a Kalman filter seems like overkill.

As @MBaz says, one model you could use is:
$$
r = V + n
$$
which has no time-dependent component. Making a Kalman-amenable model out of this might look like:
$$
V_{k+1} = V_k + m_k\tag{1}
$$
$$
r_k = V_k + n_k\tag{2}
$$
and we'd use the 14V nominal voltage to say $V_0 = 14$.

However, we are given no information about $m_k$ - how the true voltage might vary. We might also want to augment the state (perhaps the variation if $V_k$ depends on the derivative of $V+k$?) and so include a state transition matrix in front of $V_k$ in (1). If we did augment the state, we'd also probably want to add an output matrix on (2).




  Could you give me an insight about the initial covariance? Can it be reached from the information given?


Generally, I choose the initial state covariance to be VERY large. This is because I am generally given zero information about what it should be, as in this case. In this case, "VERY" might mean 30 (ten standard deviations).