Sorry you're feeling like this - seeing the GP for treatment is a really positive step though.
The side-effects depend on (a) the particular drug and (b) the person taking it. Not everyone gets the same side-effects.
I was on Citalopram for about a year. For the first couple of weeks I had a few side effects (nausea, constipation, diarrhoea, night-sweats, wooly-headedness, loss of appetite etc) but then settled down well. I had two long-term side-effects - one was an intolerance to sunlight, t other was a feeling of having an electric shock in my head every time I turned it. That's apparently an unusual side-effect with Citalopram though.
Other common side-effects are a dry mouth, headaches and shakiness. They are usually short term and disappear after the first week or two.
Some people experience more extreme reactions to some ads, and they can actually feel more depressed, more anxious, or start having panic attacks. Again, these aren't common and can soon be solved by being prescribed a different form of ad.
The SSRIs are more easily tolerated than some of the older forms of ads, and the side-effects are generally reckoned to be fewer and shorter-lasting.