  A detailed study of the electronic structure and magnetic configurations of
the 50 % hole-doped double layered manganite LaSr$_2$Mn$_2$O$_7$ is presented.
We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons {\it
(i)} significantly modifies the electronic structure, magnetic ordering (from
FM to AFM), and interlayer exchange interactions, and {\it (ii)} promotes
strong anisotropy in electrical transport, reducing the effective hopping
parameter along the {\it c} axis for electrically active $e_g$ electrons. This
findng 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 LaSr$_2$Mn$_2$O$_7$ is found to
be energetically more favorable with U $\geq$ 7 eV. 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 $e_g$ state sub-band widths.
