to start with, figure out what exactly needs to be done and why and put it in writing
The first step would be to write down exactly what you're looking for this app to do and any other pertinent requirements (needs to work offline, speed requirements, hardware to support, etc.).
What you're describing so far sounds like a service provided by major online map providers such as Google, Bing, etc. If you have an address, you just key it in as a destination, and then if you have the fire station pre-filled in as your departure point, you'll get map directions generated right there. Is this what you're interested in? If so, then you can just open a browser when you receive a message and go to Google Maps or to the maps app directly from the phone and do it there.
If some of the details that you've provided are hard requirements (text message being an incoming SMS, .xls data store, precise data requirements, zero downtime tolerance, very fast response times, etc.) then the best place to start would be finding an acquaintance who can write phone apps (maybe some kid from a local high school or college?) and explain/show what you want done to them in detail and have them estimate how complex this work would be. After than you either hire that person to do the work or pressure them to do it for you as community service
If you don't have the right people in your network or they're unwilling to do the hard lifting, then you should reach out to a contractor and have it done commercially. It's the same approach: explain what you need done exactly, then get an estimate, etc. Based on what you describe, it shouldn't be too complicated or expensive.
I don't recommend you do it on your own unless you have tons of free time and/or are specifically looking to learn how to write apps. You'll end up spending a lot more time than this project costs on the market, and if you're not really into it, it might seem quite frustrating for what seems like should be a very simple thing.