[MENTION=13209]Wanderer[/MENTION]: I'm not an ISTJ but I'm close, and I'm female, and I have lots of INFJ friends and family members, so I'll lend my voice in here

Not sure if you still need the advice still, since you asked the question several months ago, but whatevs!
I'll focus a lot on what you should expect as well as what she expects, to avoid future relationship resentment on your part.
Amber's right that ISTJs like consistency, and I'd say also that if they're like ESTJ females then they care a lot about thoughtfulness, stability and honesty. Elaboration:
1: Thoughtfulness
xSTJ women may not be "romantic" in the traditional sense, especially compared to NFs, but we are very very thoughtful and we express our love that way -- meaning, that's what we expect from our partners too. We remember what's going on in your life, and we try to do what we can to be supportive and to make the bad things better for you by showing that we're there, and we see a lack of that behavior from a partner as meaning that they don't care as much about us as we do about them.
2: Stability
Partners rely on each other, and just as you probably expect to be able to rely on your very steady and level-headed ISTJ when something comes up in your life, so too does the ISTJ expect to rely on you. A lot of people don't really realize that about xSTJs at first, because we seem so in control all the time -- so if/when your ISTJ has a very vulnerable moment (which may be disguised under a veil of sarcasm and/or self-deprecation), she does want you to be a supportive friend and loving partner and comfort her in a grounded way. (Those moments of opening up are often my personal litmus test for friendships; if my opening up doesn't make the person awkward --which it often does-- then I can become even deeper friends with them, but if it is awkward, then I know to not open up to them for a while.) Emphasis on "grounded" because if an xSTJ is freaking out about something and are talking to you about it, it's because they don't want to be freaking out anymore; they want to hear a reason why they don't need to be freaking out, or, if not that, then something they can do to fix the scary thing that will definitely work. So: be there for them, be calm and steady when they aren't, just as they're calm and steady for you when you aren't.
3: Honesty
Healthy relationships can't be healthy if you're afraid to tell your partner the truth. If an ISTJ asks if her dress makes her look fat, she might actually appreciate your negative but honest answer (as long as you're not in public at the time!). And if an xSTJ female complains about something really upsetting that's going on in her life, it's pretty likely -- much more likely than if an NF was complaining about it -- that she wants advice or input. If ISTJs are like ESTJs, we try to solve our most serious issues internally before we, as a last resort, try and get help from elsewhere.