Hypothesis and significance level?

I'm kind of lost on what I guess is the basics of this hypothesis testing.

In my homework assignment, I am asked to find data on the internet. With that data, I am to:
1. Formulate a null and alternative hypothesis based on a research question that you develop
2. Conduct a test of the hypothesis at a specified level (such as alpha = .05) of significance
3. Summarize your conclusion

I chose to collect data on the gas mileage of SUVs. I wanted to find out if the highway mileage was at least 18 mpg. My null hypothesis is that the mileage per gallon is equal to18. My alternative hypothesis is that the mileage is less than 18 mpg. I took data from www.fueleconomy.gov on the gas mileage of similar SUVs. I checked 50 vehicles and found the mean of 17.7. At the .05 significance level, is the gas mileage less than 18?

Ho: = 18
Ha: < 18

Am I going in the right direction? Should I be using null/alternative that use a probability of the gas mileage being =18/<18? Do I need to include the number of SUVs in my sample? It took me forever to figure out what data I was going to collect and to apply it.