All living things need food to survive and orchids are no exception! As a general rule, a weekly/weakly feeding is ideal. This means a weak solution of fertilizer every time you water. Always remember the general rule: Only water when the medium is just to the dry point. This means making sure the orchid is dry throughout its medium, not just by feeling from the top. The ideal way to fertilize is to water thoroughly with water and then water with your diluted food mixture to be sure the roots are absorbing it and the food is not just draining right out of the pot.
We use Dyna-Gro fertilizers here at the shop and sell Dyna-Gro products for your orchid needs.