I'm not a wine-o by any stretch but a nice sweet-dry red goes great with beef. A good bottle that wont set you back too much is House Red, its like $12 a bottle. As far as sides, a baked potato is very classic and you can dress it anyway to fit your individual tastes. I would also recommend looking at a farmers market or produce stand / not grocery store for some fresh veggies you can steam. They are incredibly delicous and steaming them will add some bright colors and freshness to the plate. Almost everything should be in season now, so there are bound to be some deals. I'd stay away from the more niche veggies like asparagus unless you know she likes them.
I hope that helps!