I got a poor back posture and I slogs sorry that I can't spell it alot how do I...

[Pedro C]

...strengthen it? I would like to strengthen my back and I know that there are three parts of the back the top the middle and the lower back and I used to go to a bailey total fitness gym but now I'm not able to afford it.I have a 15 pound of dumbell but I'm afraid to injure myself and not doing the exercise properly.I don't know if stretching exercise works I think that's more for flexibility and I don't want to injure my lower back either in the pass I did alot of lower back exercises and I also do got a curl lower back