For small stuff, pick one thing and try to do it every day for three weeks. Once it is a habit, it should have momentum of its own. Rinse and repeat. (Keep a log book or you will forget and feel like nothing has changed.)

Negative thoughts can be more complicated. I will recommend you start by drinking a glass of water, having a bite to eat and/or taking a short nap. Negative thoughts can have multiple causes, but they tend to be more persistent when you are hungry, tired and thirsty.

For "bad habits," it generally works better to find a better way to meet your needs than to try to deny yourself something. For example, trying to eat healthier is generally a better approach to weight loss than restricting how much you eat.