Ah the worry's the worst part but its not too bad. Its different for everyone.
Your frontal lobe is very sensitive to changes in serotonin and it also controls things like facial muscles with the jaw. So you might get a bit of slight grinding at first while you're adjusting. Also, you can feel a bit nauseous for the first couple weeks but i didn't get that personally. It might effect your bowel movements a bit, and you'll feel a bit fuzzy in general.
Btw you will have very vivid dreams for a bit, it's pretty cool actually but they can be pretty lucid. You're appetite will change a bit too, and you might start yawning at first/feeling awake at the same time.
Thats the general symptoms you're most likely to experience above most, they have to have that massive list on the pack just for disclosure, a lot of them are rare but they legally have to put them on so don't freak about it lol. I'd say if you can, take a bit of time off to adjust, keep yourself hydrated for the dry mouth (goes away pretty soon) and get lots of sleep.
All these things go away and are temporary, it usually takes 2-4 weeks for them to go away totally and about 4-6 to start feeling the accumulative effect of the SSRI. You should notice a difference sooner than that anyway but thats the general consensus :)
Depends what med you're on but lemme know what it is if you wanna know, i've been on citalopram, ecitalopram (lexapro) and mirtazapine. Hope this helps