Go to a chiropractor
Lay on your back on a ball
Hang from a bar, with your hands or upside-down with gravity-boots
Wherever your spine is bulging out, workout the muscles on the bulging side
Straightening your posture takes time because it involves both physically changing your body as well as mental bad habits. So, it's not something that will happen overnight - so you have to give it a lot of time. Taken me years, to be honest, and I'm still not where I wanna be (although somewhat close). But the pay-off is good and worth it. Nothing spells a loser like a slouch.