According to newton's first law of motion an object tends to remain at rest or in uniform motion until and unless and external forces acts on it so suppose a dog is sleeping and then suddenly it wakes up and starts walking so it has come into motion without any external force acting on it how

Here, the dog is using his muscular force to get up and walk. So, this force applied by a dog is kind of external force.

 

To make it more clear for understanding, let us consider one more example -

Consider that there is one paper placed on a table. This paper will not move from its place unless and until any external and unbalanced force acts on it. Now, if you switched on the ceiling fan then the this paper will flies off from the table due to the air which acts as a unbalanced and external force on the paper and hence, the paper is set in motion. 

 

Hope you have understood the concept.