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.