The initial treatment recommended for foot tendonitis is called the R.I.C.E. method – Rest, Ice, Compression and Elevation.  Ice should always be wrapped in a towel, not applied directly to the foot.  Also, don't leave the ice on your foot for more than 20 minutes at a time.  You can apply the ice several times a day, though. You can also take over-the-counter anti-inflammatory medication, like aspirin or ibuprofen, if needed.

If that doesn't help, you really need to see a podiatrist or an orthopaedic specialist.  The doctor may recommend special orthotics inserts for your shoes, corticosteroid injections, or in some cases, even surgery. 

Here's a link to more info on foot tendonitis:  http://www.podiatrynetwork.com/r_tendonitis.cfm