Help me to stop slouching!

I have very poor posture. I've always been self concious, so i kept me head low and my chest in.

Most people have been told to stand up strait since they were young, but i never took to it because it wasn't the natural thing to do.

I've been slouching for years. I'm trying to learn how to stand up straiter to try to reverse and avoid any current and future damage to my back, but it is very difficult.
I've always slouched without realizing it, so it takes concious effort to try to stand strait, but i don't know if i'm doing it right.

Can you please give me some tips so i won't be hunched over in my later years (and don't say "go to a doctor" because i'll try to go to one later)