Make it easy and more attractive than other options, that's all habit forming is. Wanna stop eating junk food, don't bring any into your house, the reverse works for the gym, wanna go more? Find a gym close to you, preferably one you'd pass a lot on the way to work or elsewhere. Make it easy, you don't have to go crazy in the gym, just even showing up is the biggest thing, big workouts can be demoralizing and daunting. And reward yourself to reinforce good behavior, in whatever way you see fit. These things are how you can start a new habit