Often found in hardware and building supply stores. Comes in a wide range of types and thicknesses, and may have surface treatments. See Painting on Metal: An Introduction for more detail.
At minimum, scuff the surface, degrease with isopropyl alcohol, then use a commercial DTM (Direct to Metal) or Bonding Primer suitable for aluminum and able to accept waterbased paints. Always follow the manufacturer’s recommended preparation.
Since commercial primers may yellow, applying 1 or more coats of acrylic gesso on top of the primer will provide a whiter ground.
Oils and Acrylics can be painted directly on this preparation.
For watercolors and dry drawing media, an additional acrylic ground appropriate for each media should be applied before painting.