I am going to start taking citalopram (celexa) tomorrow for depression, how can i expect to feel?

Hopefully nothing: Generally, you should not feel any changes within the first two weeks of taking an ssri. Sometimes people will get mild side effects like nausea, upset stomach, constipation/diarrhea, dry eyes/mouth. Rarely, there may be sexual side effects. Let you doctor know if any of these occur in a way that is intolerable. After two weeks (wks 2-6), you should start to feel benefits.

Celexa (citalopram): Like other antidepressants, it takes 4-6 weeks to see full effects, however, early side-effects can occur such as nausea / headaches.

Don't stop abruptly: All above are good answers. An additional caution is to not stop taking abruptly. If you need to go off this med, a gradual tapering, under supervision, is indicated so as to not experience unpleasant withdrawal symptoms.