String theory much like Gauge theory is not fine tuned to any set of observed phenomena and for the most they don't make numerical predictions. What they do predict is the general shape of cross-sections for different particle species and corrections to them in the case of Gauge theories and much in the same way one can work out various String scattering cross sections. In the case of Gauge theories this is how you determine which of them fits the measured cross section best. In the case of String theories this failed when it was still believed to be a theory of the strong interaction. It eventually was realized that the string scale lS is related to the Planck lP by

lP = lS * gS^{1/4}

where gS is the vacuum expectation value of the Dilaton. It is believed to be of order one, so lS is ~10^35 meters. The current accessible length scale at the LHC ~10^18 meters, so you have 17 orders of magnitude between them and in particular no reason to expect the string scattering cross sections to relate to anything we can observe directly.

What is found instead is that string scattering amplitudes assemble in a way that predict that the massless string excitations form Field theories already known, like type IIB supergravity, IIA supergravity, SO(32) and E(8) x E(8) coupled to supergravity and a few others. What is striking about those initial calculations, is that from field theory arguments those field theories are the only supersymmetric ones in 10 dimensions incorporating gravity.

Perturbative String theory in its current form makes most of the model input data part of the "vacuum geometry", much in the same way Einstein did with general relativity. String theory in addition has the advantage that it gives a consistent perturbative description of quantum gravity.

String phenomenology, that is the study of String theory with the purpose of making low energy predictions is still in its infancy. Its value at the moment is primarily in coming up with consistent extensions of the Standard Model, the alternative of conservative step-wise refinements (conjecture additional particles to exist and their coupling mechanism to the known particles, calculate cross sections and hope that they aren't ruled out by experiments yet, for example cause existing particles to decay that aren't really supposed to, like the proton) is still the majority approach. Dark matter and other theoretical considerations tell us however that the Standard Model is incomplete.

The attractiveness of the string theory approach is that in some ways it is more restrictive as you have to satisfy additional geometric constraints, whereas in standard gauge theory the only consistency constraints are unitarity and locality, which predicts that fundamental particles will have spin 0,1/2,1 and 2 and renormalizability which restricts you essentially to Yang-Mills theory, plus some extensions (Supersymmetry among others). What isn't fixed is the gauge group, the precise way Fermions couple to each other etc. Some of those can't be fixed because things like the coupling constants aren't actually constant but dependent on the energy scale (they are related by the renormalization group).

Most likely if we find some new particles at the LHC it will be possible to give them a low energy effective field theory description (maybe involving supersymmetry), if we don't find things like extra dimensions. In other words it is unlikely that we are forced to consider string theory in order to understand physics at the LHC scale, its value is that it potentially has better conceptual tools to come up with super-symmetric extensions of it.