Get the Portland bike route maps, you can pick them up at most bike shops, order them free from the city's website, or download a PDF. http://www.portlandoregon.gov/transportation/39402

It shows the best routes.

I haven't ridden from Univ Portland to Sellwood, but I've ridden most of the roads on the way. Williamette is fine, Greeley is too, and so is Interstate. "Fine" meaning there is a wide enough bike lane. That's all you need, plus some basic defensive riding. Portland drivers and cyclists co-exist pretty well, I routinely commute with cars a couple feet alongside. Then you can get on the bike/pedestrian path by the river and take it all the way to Sellwood. Seems like a good commute route overall.

I'd rather ride Greeley-Interstate than the alternative you mentioned, because there are fewer intersections. I think most of the dangers riders face are at intersections (right hook, left hook, red light runners, etc).

You should have good lights and reflective bag/jacket if you're riding in the dark. Lots of threads here and in the Electronics forum about lights. I use a four LED Knog Blinder on the seatpost and a $15 single red LED blinky on my helmet. On the front, at least have a flashing white multiple LED light, like the PDW or Planet Bike ones, I think that is the minimum.

If you're not experienced riding on streets and bike lanes, ask here for tips, also see if the BTA (Bicycle Transportation Alliance) is giving classes in riding safety. Also, the bike shop near you, Sellwood Cycle, should have some tips and local info; I hear it is a good shop.

Welcome to Portland. Look for info on Pedalapooza which starts in a couple months, tons of bike rides and events, fun and a good way to see different parts of the city. Also join a welcome ride with the Portland Wheelmen club, any question you have about local riding can probably be answered by the club members.