  In this paper, we extend to the function field setting the heuristics
developed by Conrey, Farmer, Keating, Rubinstein and Snaith for the integral
moments of L-functions. Also, we adapt to function field setting the heuristics
first developed by Conrey, Farmer and Zirnbauer to the study of mean values of
ratios of L-functions. Specifically, we obtain an asymptotic formula for the
integral moments and ratios of the quadratic Dirichlet L-functions
$L(s,\chi_u)$ over the rational function field $\mathbb{F}_q(T)$, when $q$ is a
power of 2 and over a given family. As an application, we calculate the
one-level density for the zeros of these L-functions.
