When you think about it, it's not that hard. Little things that make you happy, like petting a kitten, talking to your friend or watching good movies is something that makes you feel good. You can be grateful for everything.
Start small. Pick something completely ordinary to you to be grateful for in your immediate space. It could be your bed, a t shirt that it comfortable, or even just the weather today. It may feel silly but it makes it easier to start the practice of gratefulness.
Looking at the the things that make you happy and looking at the world around and try to see the possibilities because that's somethings you can be grateful for