They don't need much and over applying can lead to lack of subsequent flowering so best to use a light hand. Most sources will recommend an orchid specific fertilizer but I just use my regular Dyna Gro houseplant fertilizer at the maintenance or well diluted level. Once every 2 weeks or so at half to quarter strength during the vegetative stage, once a month or so when in bloom and skip entirely during the winter months.

Best to make sure the plant is well watered before applying the fertilizer. This will protect against root damage if the fert concentration may be too high.

What watering instructions do you have and what is the media its potted in?

tj