I don't think there's a so-called best branch of engineering for a girl. You need to go with your interests first, and then there are other factors that may influence your decision, such as working environment and pay. I think currently computer engineers get paid the most (for a starting salary), and they can work on integrating hardware (such as circuit design and power supply to a computer) and software (such as writing programs and operating systems).
Electrical and electronic engineers come in second in terms of starting salary. Electrical engineers can work on a variety of jobs from designing the power grid of a city to a microchip for a cell phone, while electronic engineers (sometimes considered to be a subgroup of electrical engineers) typically work on smaller-scale products such as computers, digital cameras, GPS system inside a car, etc. Civil engineers typically work with architects on the structures on buildings and bridges, so the engineering concepts are easier to visualize and the results easier to see. The first 3 fields of engineering generally won't require you to be outdoors (maybe with the exception of working on a power grid) whereas civil engineering may involve occasional trips to construction sites.
Keep in mind that even within a single discipline, the jobs can be very different. Take advantage of chances in school where you can visit engineering companies or contact engineering alumni to find out more about the daily tasks of their job. If you are concerned with how many female classmates you'll find as you study engineering, civil engineering generally has about 30-40% girls whereas the other three have about 10-20%, but those numbers are slowly rising.