Check the properties of the raster first if there is an existing projection.
right - click the raster name on the table of contents > properties
if there is none and you know the projection, go to toolboxes > data management > projections and transformations (p&t) > define projection.
If you don't see it, then you may have to go inside raster : p&t > raster.
then select from the given geographic coordinate systems (usually the coords are in degrees) or projected coordinate systems (coords in meters or some other unit.)
then execute.
if there is already an existing data projection, then do this:
go to toolboxes > data management > projections and transformations (p&t) > raster > project raster.
you can choose the desired coordinate system to project the raster, as well as the geographic transformation/s to be used.
hope this helps. =)