Question

according to Newton's first law we can't walk by ourselves so how do we walk without any external force acting on us

Answer 1

The moving-forward movement can finally take place thanks to BOTH friction force and the work of your knee and hip articulation. Its the frictional force. When you walk your feet push the ground backwards and according to Newton's third law, the ground pushes you forward.

Answer 2

Newton’s first law of motion states that there must be a cause—which is a net external force—for there to be any change in velocity, either a change in magnitude or direction. An object sliding across a table or floor slows down due to the net force of friction acting on the object. But on an air hockey table, where air keeps the puck from touching the table, the air hockey puck continues moving with a roughly constant velocity until a force acts on it—like when it bumps into the side of the table.