Investing beginnings for 18 year old

[Antonio Jimenez]

Hello, I'm 18 years old going to community college and working part time. I plan on buying my first property before I turn twenty and was wondering on your thoughts on how I should be saving or investing my money. At the moment i have 6k saved in cash that I have with 1k in a student account at chase and 5k in cash. I work construction on the weekends and make 300 a week. I am wanting to invest into a index fund while i build up some cash to but a down payment of 20% on a triplex or fourplex to house hack in the metro-phoenix area. 

So at the moment I have 6k while making 1200 a month in cash as well. Since I am paid under the table I was wondering if anyone could tell me how i would be able to get a loan without any credit?

Thank you for taking the time to read this and help me on my way to learn more