The answer to your question is largely personal. However, there are a couple things you may want to consider:
- How strong am I?
- How thin do I want to be?
- How do I want to feel?
Take a look at this link with visual references for body fat percentages. Keep scrolling down for the women. What you want to look like is a personal matter, but those give you a visual reference so you can see when it happens to you.
But I do want to drive home one point. Look at the image below (I know it's a man):
The same body fat percentage will look differently once you've hit it based on what's underneath the fat. That's where the muscle comes in. I'm not saying you have to lift weights, even though it brings a smile to my face every time I see another woman lifting seriously in the gym. The point is, without some form of exercise, you may look like the poster child for anorexia where your knees are wider than your thighs due to the lack of muscle.
The bottom line is that you can work on both at the same time. It will probably be faster to do what you want at the same time, but if you slowly melt off the fat onto a fit frame, you'll probably be a lot more motivated to get to your end goal.