I worked in property management in a apartment complex and our maintenance team switched out the locks every time a tenant moved out. 

It's a great time to get started.   

I've done this several times, it can be a little awkward sometimes but most tenants understand.

We stuck it out, paid our dues on time.

However, my credit is not good and I'm having a difficult time getting anyone to rent to me.

Be part of the "Class Action!" 

If I were 19, I’d work for a multifamily developer or an big time investor.

Denver is very expensive for a new guy who doesn't have loads of capital so a market like Missouri for example is intriguing for far cheaper real estate but then Id either have to move, improve the property remotely or simply buy and hold as a rental for X time. 

It just takes a little time/effort to get the right information to stick.1.

Whether you spend time networking, reading books, watching videos or connecting on bigger pockets or if you spend cash hiring a mentor, maybe partnering up with or doing free intern work for a real estate investor; whichever route you decide to go investing in yourself is one of the best investments you can make.