That's right, every thought you have contributes to your current physical state. If you believe you are strong capable and fit, you'll be more likely to eat well, exercise and take care of yourself, if you believe you don't matter, you're weak, and that exercise doesn't benefit you, well...guess what? your behaviors will reflect this and lead you to do all the things that in fact will make you weaker and more out of shape. So.....What's the first step to becoming what you'd like to be? THINK ABOUT IT. That's all. Just practice a few thoughts a day, say it out loud if you want to! 
Try these thoughts for starters "I am strong" "I can do this" "Every bit counts" 
Don't change your whole life at once. Drastic measures usually lead to drastic failures, take it step by step, start thinking positively, making small easy changes, and before you know it, you will surprise yourself.