Start off with house hacking (Buying a multifamily and living in one of the units), this will allow you to have others paying your mortgage and will get rid of one of the largest monthly expenses you're going to most likely have (Will save you anywhere from $700-1500 each month, depending on what amount you borrow). All that money you're not spending on the mortgage is money that should be saved to buy the next property and it will start adding up quickly due to the decrease in expenses (Put this money into safe short term investments while it builds up so that when you're ready to go its there and theres most likely not going to be a loss.. CD's, bonds, etc).
No one is going to be able to tell you what works best for you but both rentals and flips are good options, one is just a much slower process than the other, but flips have more risks involved in my opinion.
You're off to a great start especially at a young age, just keep saving and building your network. While others around you are probably spending money likes its going out of style on things they really don't need, you're making the right choice and will be better off later on.
Good luck.