system = {x''[t] == ((((B0 k) x[t] + B0) q)/m) y'[t],  y''[t] == -((((B0 k)
x[t] + B0) q)/m) x'[t]};
initialvalues2 = {x[0] == 0, y[0] == 0, x'[0] == 10^4, y'[0] == 0};
solution = DSolve[Join[system, initialvalues2], {x[t], y[t]}, t]