After the item you spray painted is completely dry, depending on the sheen of paint used, you can apply a clear coat over spray paint. If you use high gloss, semi-gloss and you want a shiny finish, you really don't have to seal it. If using a lower sheen you can seal with a water-based sealer like Minwax Polycrylic. It comes in a spray version.  Most brand's mention on the can's label about how to seal and even sell sealers to go with certain formulas of their paints.