The habit you choose needs to be specific. It can’t be vague.
For example, don’t say that you want to exercise. Say you want to run for 10 minutes a day right after you drink coffee in the morning. Don’t say you want to drink more water. Say you’re going to drink 6 glasses of water every day.
If the habit is vague, there’s no way to know if you’re doing it. And as such, you’ll do it well on some days and not very well on other days.
You should have a measurable change — are you going to do 10 pushups, 5 minutes of meditation, floss once at night, wake up 15 minutes earlier, declutter 10 things from your home a day?
Vague habits fail. Specific ones are likely to succeed.