A detailed study of the electronic structure and magnetic configurations of the 50% hole-doped double-layered manganite LaSr2Mn2O7 is presented. We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons (i) significantly modifies the electronic structure, magnetic ordering (from FM to AFM), and interlayer exchange interactions, and (ii) promotes strong anisotropy in electrical transport, reducing the effective hopping parameter along the c-axis for electrically active eg electrons. This finding is consistent with observations of anisotropic transport-a property which sets this manganite apart from conventional 3D systems. A half-metallic band structure is predicted with both the LSDA and LSDA+U methods. The experimentally observed A-type AFM ordering in LaSr2Mn2O7 is found to be energetically more favorable with U≥7eV. A simple interpretation of interlayer exchange coupling is given within double and super-exchange mechanisms based on the dependencies on U of the effective exchange parameters and eg state sub-band widths.