Using Nehari manifold method  combined with fibring maps, we show the existence of nontrivial, weak, positive solutions of the nonlinear $\psi$-Riemann-Liouville fractional boundary value problem involving the $p$-Laplacian operator, given by

where $\lambda>0$, $0 < \gamma <1<p$ and $\frac{1}{p} < \alpha \leq1, $, $g \in C([0,T])$ and $f \in C^{1}([0,T]\times \mathbb{R,\mathbb{R}})$. A useful examples are presented in order to illustrate the validity of our main results.