There's no way you are going to be able to duplicate an American Lite Lager without good temp control and a good understanding of yeast and fermentation. For any lager, German, American, etc. you will need to ferment the beer in the 48-52 range with a true lager strain for at least 48-72 hours. You could make a great ale as well with the same recipe and use dry yeast but even then fermentation will need to be kept under control, low 60's high 50s.
I brew commercially and it is certainly not impossible to make a Lite American Lager but for beginners it is a fairly mighty challenge. Those beers have almost ZERO flavors to begin with, any fermentation mistake is going to show up very easily.
+1
I recommend that you take your beer to a local Homebrew Club. They can help you identify the aftertaste, how it got there and how to remedy it.
Welcome to the hobby. Learning is an exciting adventure!