When starting to paint for the first time, I think the best way to go is to pick a subject that you like. However, keep in mind that since you are new to painting; most likely, your final result will not be to your satisfaction.
There is a disconnection between what visually creative people like or envision in their head, and what comes out as the final product. There is nothing to be afraid of; it’s a natural thing that is bound to happen, and the only cure is to paint, paint, and paint until the hand figures out how to create what the mind wants.
That being said, as a beginner, I would recommend that you choose more straightforward images, without a ton of details, and with big shapes and simple volumes. Focus of a pleasant distribution of darks and lights, those can make any painting look awesome.
To be safe, use your own photos or images with a creative commons license. Don’t just Google an image or take one from Pinterest, those are most likely copyrighted. Go to sites like Pixabay, Morgue File, Wikimedia Commons, etc. You may also use other people’s photos, with permission. I hope this answers your question.