Probably the very first mammals that evolved from reptiles about 200 million years ago were small animals covered with fur that still laid eggs like reptiles, but their babies got milk from their mothers like mammals. Most of the mammals like this have died out now, but one survivor is the duck-billed platypus.
The duck-billed platypus is from Australia, which was so isolated from the other continents after Pangaea broke up that it preserved many older kinds of mammals like the duck-billed platypus and the marsupials like kangaroos and opossums. So looking at Australian mammals can give us an idea of what early mammals might have been like.
In many ways, a duck-billed platypus is still like a reptile. It is shaped more like a crocodile than like a cow or a dog, with its short legs spread out to the sides, and its long wide tail (though it is smaller, about the size of a pet cat). A duck-billed platypus also lays eggs like a reptile. But it is a mammal because it is covered in fur and because baby platypus drink milk from their mothers.