Are we talking bike/moped/wheelbarrow??? remove wheel, check around tyre to find cause such as nail/screw, if nothing visible then check valve/bead-to-rim for leaks by soapy water, if air coming out of valve could mean replacing/tightening core. ascertain whether its tubeless or tube type, if tubeless take to tyre depot if tube buy new tube & fit yourself/depot.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix motorcycles.