Use the word carelessly to describe something you do without thinking too hard about it. Carelessly kicking a stone down the sidewalk is fine, but carelessly driving a car down a busy street is a bad idea.
When you're not paying close attention, and especially when you're acting reckless, you're behaving carelessly. This adverb essentially means "without a care," and it can be used to mean "carefree," like when you carelessly whistle a tune as you stroll through the park. More often, its meaning is closer to "without caution or thought," like when your dog carelessly tramples your cat in his rush to the back door.