Hi Magda,
There are white South Africans as well, especially in areas like Rosebank and Sandton (not in e.g. townships though), that go about their daily lives together with fellow South Africans and friends from other races and cultures and walk from the Gautrain to wherever they have to be without any problems. Your skin colour specifically won't make you a target. A tourist here stands out if they wear big cameras around their neck, if they wear backpacks, dress very differently (e.g. "safari" clothing) or speak English with a heavy foreign accent. There are lots of tourists and being a tourist won't necessarily make you a target either. If you want to blend in and stay safe you can follow some basic rules (which is valid in any big city in the world) such as don't display valuables openly, be aware of your surroundings, don't go to areas that are specifically unsafe etc.
Sandton and Rosebank are some of the nice and safe areas that you can consider. Rosebank is generally more pedestrian-friendly than Sandton so I would suggest that area. Also, the Gautrain station there is right at the mall where there are also lots of hotels surrounding it, whereas the Sandton Gautrain station is a little bit farther from the mall and hotels in general (some exceptions). It is safe to walk around the very nice Rosebank Mall area and between the hotels surrounding it and the Gautrain station during the day or night. I would be a little more wary walking a few blocks at night on the streets anywhere in Joburg, so I would suggest choosing a hotel close to Rosebank Mall (e.g. the Holiday Inn, I've seen some AirBnb options as well and lots of others) and then you don't have to walk far. If you do arrive at night and stay farther away you can ask if your hotel has a shuttle, you can organize a private taxi or take a Gautrain Bus.
I hope this helps! :-)