Well, You can start reading a lot. Mainly about how to develop for the android platform. (like what i am doing now).
You can look up a bunch of really simple tutorials to help you set up the development tools for working on android. Then look up more tutorials on how to get a basic app coded and running. And then more reading. And more tutorials.
Also Im finding, like with anything else. The more I practice how to code, the better i understand how things fit together.
So you want a basic app that the user can
-enter amount of fuel bought(gallons)
-enter cost of fuel($$$)
-enter miles driven (Miles)
And then the app should return info to the user based off of what the user entered
-return amount/cost fuel (maybe total or maybe per gallon)
-return miles driven since last stop(maybe?)
-return MPG since last stop or calculate an average over a month?
-return amount of $$$ (cost average) per fill up.
I just threw out what came to mind in the most basic form. I could be completely off from what you actually want. You should really make a list on how you would want your app to function. Starting with what the first screen presents to the user, and basically come up with each specific step on what you would be doing in your app.
From there you can dictate what you want the screen to specifically display. You then can decide what you want to do with that info (display in spreadsheet form im guessing) and then from there if you want the app to remember for future use or to just print to the screen the info provided.
When you think of an app idea, think of how you (the user) would want to experience how the app works. Make a basic list of how things should flow together to get an Idea of how you need to program your app. Its hard to build a house without the blueprints ya?