Jets, clusters of collimated particles produced in high energy proton-proton ($pp$) collisions, are an excellent tool to study the internal structure of the proton. According to perturbative QCD (pQCD) calculations, for center-of-mass energies of $\sqrt{s} = $ 200 and 510 GeV at RHIC, jet production in the mid pseudorapidity, $|\eta| <$ 1, is dominated by quark-gluon and gluon-gluon scattering processes. These jets are sensitive to gluons in the proton with momentum fraction 0.01 $<x<$ 0.5. The STAR experiment has measured a series of jet double-spin asymmetries within the pseudorapidity region of $-1 < \eta < 2$, in longitudinally polarized $pp$ collisions, to constrain the gluon helicity distribution function in the proton. Similarly, jet cross section measurements from unpolarized $pp$ collisions are effective in constraining the unpolarized gluon distribution in the proton. In this proceeding, we will present the analysis techniques and the preliminary results of inclusive jet cross section measurements in $pp$ collisions at $\sqrt{s} =$ 200 and 510 GeV.