What are you trying to calculate?
How much gas you're going to use?
Google maps to get you how many miles from your house to your destination.
Divide the number of miles by the mpg you average. That gives you gallons.
How much gas will cost you?
Do the above, then take the number of gallons you've estimated and multiply it by $2.75 (gas around here ranges from $2.35 -$2.65, using $2.75 will give you a little cushion).
Don't know what you're average mpg is? Sorry, that's on you. I always do the math when I fill up. If you haven't, a quick google search shows you 23 city/30 highway. I would use 25mpg for calculations, then modify that as I do the math at fill up.
How to calculate mpg?
Fill up the tank. Reset your trip odometer. The next time you fill up the tank, divide the number of miles on the trip odometer by the number of gallons you've put in. Many new cars will actually have a screen showing your current mpg and your average mpg.