A great way to start your motivation is to make a start-ritual. This ritual has to be so easy that you can't say no to it. Meaning you shouldn't need to be already motivated to start your ritual. It has to be so easy that you can do it at any time without problems.
For example, if you want to join a yoga class, your start-ritual could be preparing your yoga mat and clothes. Or, if you're going to go for a power walk, your start-ritual could be filling your bottle with water.
Both of these rituals are so easy to do, you can't say no.
You know now that motivation typically comes after starting. Therefore, your motivation ritual needs to be amazingly easy to begin. And once you have done your start-ritual, you will find it easier to sign up and drive to the gym to join that yoga class.