First thing I would do, make sure key is out of ignition, nothing on....
Then disconnect the neg bat cable first, then the positive.....make sure there is no corrosion on the cables, or the bolts....clean if necessary.....
Check pos battery cable connection at starter, that it is clean/tight......check neg battery cable connection on block, that it is clean/tight..... apply dielectric grease to the contact areas of cables and attach positive cable first to battery, and then the neg cable....make sure tight, but don't over tighten....
If problem still exists, could be a bad connection somewhere, or an ignition switch problem.....