Hi, I live in Sapporo. It's really a beautiful place, I'm sure that you'll love it.
So, the city is really easy to get around in. It's based on a grid system with each block named something like "North 11 West 3" (Kita-11 Nishi-3 of course) (I used to live there until I moved
). The subway is convenient, always on time, and easy to navigate (only 3 lines, so it's not like Tokyo's subway).
If you don't know any Japanese, remember Kita is North, Minami is South, Higashi is East and Nishi is West. It'll help with subway stops and addresses, although street signs are usually in kanji.
I'd rely on your girlfriend's advice on what to see and such. Advice on stores depends on what you want to buy. Sapporo Station has several major shopping areas and malls that sell nearly anything you'd want. Bic Camera (major electronics store) is immediately adjacent to the south-east. Yodobashi (the other major electronics store) is about 2 blocks west of the north entrance to Sapporo Station.
Most of the nightlife in Sapporo is in Susukino (take the subway, bike or walk). It's on the Nanboku (South-North) Subway Line, 2 stops south from Sapporo Station. There are -hundreds- of bars. Be careful of anything that seems to be sex-related, you probably won't be welcome there. Viva is my favorite ex-pat bar in Susukino, but it's really small and hard to find. In the other direction, there's nightlife around Kita-24, 3 stops north of Sapporo Station. It's usually cheaper than Susukino.
I don't know your age, but there are also a couple of student bars I like - Chitlin' Circuit Freaks Shot Bar (whoa - what a name) at Kita-17, Nishi 4 (small, hard to find) and Shooters' (a few blocks south and one east - sorry I don't know the exact address).
Have fun in Sapporo - it's a beautiful place and a beautiful time of year.