This paper is concerned with the following nonlinear fractional boundary value problem: $$\left \{ \textstyle\begin{array}{l} D_{0+}^{\alpha}u(t)+f(t,u(t))=0,\quad t\in(0,1), u ( 0 ) =u' ( 0 ) =0,\qquad D_{0+}^{\beta}u(1)=\int _{0}^{1}D_{0+}^{\beta}u(t)\, dA(t), \end{array}\displaystyle \right . $$ where $2<\alpha\leq3$ , $0<\beta\leq1$ are real numbers and $\int _{0}^{1}D_{0+}^{\beta}u(t)\, dA(t)$ denotes a Riemann-Stieltjes integral. By means of monotone iterative technique and some inequalities associated with the Green function, not only the existence of nontrivial solutions or positive solutions is obtained but also iterative schemes for approximating the solutions are established, which start off with simple functions, which are feasible for computational purposes. An example is also included to illustrate the main results.