Where do I go from here?

Hi!
So you are just 18 years old and already trading that Is great.

First I would like to remind you that very few people are actually making money out of day trading, however on saying that it can be done. Takes patience and education. For me education was the key.
What I suggest that instead of back testing and looking for a system you would be better off studying support and resistance. There is a indicator in Futures io for Ninja Trader 8 called superdemzones that will place support and resistance automatically on your charts.
Secondly have a understanding of volume profiling on the vertical axis their is quite a lot of information on the topic now and some profiling indicators in Futures io. I use Rancho Denero profile indicators you have to purchase them

Thirdly get some understanding of the Elliott wave theory. I trade using the Elliott wave theory I have a subscription with Elliott Wave International there is educational resources on there web site if you are interested.

That is the direction I have taken with trading, I only trade the one indices, I suggest start with the Emini. lot of information about the Emini.

I hope this will help and best wishes