Depends on what system you are using right now and the system you plan to use in the future. In general bash and powershell only allow you to automate scripts on your system, this can be useful for repetitive tasks but have no overall direct application in building a web app or mobile app as such. Learn either if your goal is to have better control over your native system. I started with bash scripting, so I have a personal bias towards it. If you use a linux system, go ahead and learn bash shell scripting, it will help you understand programming concepts and general linux system administration.
But the web app is a far off goal if you have yet to start programming, and by the time you have all the other tools to make one, you would also very likely be proficient enough to use either with minimal effort.