I've wondered about this a lot. I'm guessing that, being a person who looks nothing at all like an Italian and given the fact that I speak a barbarian language that I'd be enslaved. Hopefully I'd manage to make a good impression on my dominus, enough for him to let me tinker with some blacksmith's tools and chemicals. After I show him a bunch of random crude gadgets (crude ass mechanical farm equipment, electrical doodads that shock people, doornobs) that I'd build (engineering student here), hopefully I'd be able to make blackpowder muzzle loaders.
Once my master was rich enough from Rome's new arms industry, hopefully I'd be able to earn my freedom. Couldn't be a full citizen of course, but hopefully my techno-sorcery would be enough for me to be seen as "That crazy redheaded barbarian fuck who lives at the edge of the city in the villa with magic flameless lights".
I'd try to create penicillin or basic antibiotics, maybe found a hospital, but I only know basic medicine. Maybe I'd draw maps of the world. I'd certainly warn the Romans about the monstrous yellow peril that's going to come charging out from the steppe in the next thousand years, but I'm sure they'd be more worried about the Germans until then. Of course, Romans with muskets....
